Automatic merge from submit-queue

Add "make help" to list all make targets and help info

As discussed in [PR#29320comment](https://github.com/kubernetes/kubernetes/pull/29320#issuecomment-234420145), add a `make help` to make the use of `make` easy. Though it works well on my Ubuntu now (see the output as below, not sure if it still works on other systems), I believe the scripts are somewhat ugly, so, any suggestion for optimization is welcome.

BTW, I intended to list targets by groups, but it's hard to do that automatically. So I just list them in alphabetical order. I think this may be enough.

There are still some TODOs (also mentioned in the Makefile):
1. make EXCLUDE_TARGET auto-generated when there are other files in cmd/
2. should we exclude the target "cmd/libs" but include "cmd/libs/go2idl/*"?
3. should we let `help` be the first/default target? It will show the help when we just type `make`.

1 and 2 are to exclude the invalid targets generated by `$(notdir $(abspath $(wildcard cmd/*/)))`:
- OWNERS is just a file, it can't be a target
- libs itself cannot be built

/cc @thockin @jfrazelle @MHBauer @dims 

Output:

```
root@vm:/home/paas/zxp/code/k8s/fork/kubernetes# make help
--------------------------------------------------------------------------------
all
# Build code.
#
# Args:
#   WHAT: Directory names to build.  If any of these directories has a 'main'
#     package, the build will produce executable files under _output/go/bin.
#     If not specified, "everything" will be built.
#   GOFLAGS: Extra flags to pass to 'go' when building.
#   GOLDFLAGS: Extra linking flags passed to 'go' when building.
#   GOGCFLAGS: Additional go compile flags passed to 'go' when building.
#
# Example:
#   make
#   make all
#   make all WHAT=cmd/kubelet GOFLAGS=-v
#   make all GOGCFLAGS="-N -l"
#     Note: Use the -N -l options to disable compiler optimizations an inlining.
#           Using these build options allows you to subsequently use source
#           debugging tools like delve.
---------------------------------------------------------------------------------
check
# Build and run tests.
#
# Args:
#   WHAT: Directory names to test.  All *_test.go files under these
#     directories will be run.  If not specified, "everything" will be tested.
#   TESTS: Same as WHAT.
#   GOFLAGS: Extra flags to pass to 'go' when building.
#   GOLDFLAGS: Extra linking flags to pass to 'go' when building.
#   GOGCFLAGS: Additional go compile flags passed to 'go' when building.
#
# Example:
#   make check
#   make test
#   make check WHAT=pkg/kubelet GOFLAGS=-v
---------------------------------------------------------------------------------
clean
# Remove all build artifacts.
#
# Example:
#   make clean
#
# TODO(thockin): call clean_generated when we stop committing generated code.
---------------------------------------------------------------------------------
clean_generated
# Remove all auto-generated artifacts.
#
# Example:
#   make clean_generated
---------------------------------------------------------------------------------
clean_meta
# Remove make-related metadata files.
#
# Example:
#   make clean_meta
---------------------------------------------------------------------------------
cross
# Cross-compile for all platforms
#
# Example:
#   make cross
---------------------------------------------------------------------------------
federation-apiserver
federation-controller-manager
genfeddocs
# Add rules for all directories in federation/cmd/
#
# Example:
#   make federation-apiserver federation-controller-manager
---------------------------------------------------------------------------------
gendocs
genkubedocs
genman
genswaggertypedocs
genutils
genyaml
hyperkube
kube-apiserver
kube-controller-manager
kubectl
kube-dns
kubelet
kubemark
kube-proxy
kubernetes-discovery
libs
linkcheck
mungedocs
# Add rules for all directories in cmd/
#
# Example:
#   make kubectl kube-proxy
---------------------------------------------------------------------------------
generated_files
# Produce auto-generated files needed for the build.
#
# Example:
#   make generated_files
---------------------------------------------------------------------------------
ginkgo
# Build ginkgo
#
# Example:
# make ginkgo
---------------------------------------------------------------------------------
help
# Print make targets and help info
#
# Example:
# make help
---------------------------------------------------------------------------------
quick-release
# Build a release, but skip tests
#
# Example:
#   make release-skip-tests
---------------------------------------------------------------------------------
release
# Build a release
#
# Example:
#   make release
---------------------------------------------------------------------------------
release-skip-tests
# Build a release, but skip tests
#
# Example:
#   make release-skip-tests
---------------------------------------------------------------------------------
test
# Build and run tests.
#
# Args:
#   WHAT: Directory names to test.  All *_test.go files under these
#     directories will be run.  If not specified, "everything" will be tested.
#   TESTS: Same as WHAT.
#   GOFLAGS: Extra flags to pass to 'go' when building.
#   GOLDFLAGS: Extra linking flags to pass to 'go' when building.
#   GOGCFLAGS: Additional go compile flags passed to 'go' when building.
#
# Example:
#   make check
#   make test
#   make check WHAT=pkg/kubelet GOFLAGS=-v
---------------------------------------------------------------------------------
test-cmd
# Build and run cmdline tests.
#
# Example:
#   make test-cmd
---------------------------------------------------------------------------------
test-e2e
# Build and run end-to-end tests.
#
# Example:
#   make test-e2e
---------------------------------------------------------------------------------
test-e2e-node
# Build and run node end-to-end tests.
#
# Args:
#  FOCUS: Regexp that matches the tests to be run.  Defaults to "".
#  SKIP: Regexp that matches the tests that needs to be skipped.  Defaults
#    to "".
#  RUN_UNTIL_FAILURE: If true, pass --untilItFails to ginkgo so tests are run
#    repeatedly until they fail.  Defaults to false.
#  REMOTE: If true, run the tests on a remote host instance on GCE.  Defaults
#    to false.
#  IMAGES: For REMOTE=true only.  Comma delimited list of images for creating
#    remote hosts to run tests against.  Defaults to a recent image.
#  LIST_IMAGES: If true, don't run tests.  Just output the list of available
#    images for testing.  Defaults to false.
#  HOSTS: For REMOTE=true only.  Comma delimited list of running gce hosts to
#    run tests against.  Defaults to "".
#  DELETE_INSTANCES: For REMOTE=true only.  Delete any instances created as
#    part of this test run.  Defaults to false.
#  ARTIFACTS: For REMOTE=true only.  Local directory to scp test artifacts into
#    from the remote hosts.  Defaults to ""/tmp/_artifacts".
#  REPORT: For REMOTE=false only.  Local directory to write juntil xml results
#    to.  Defaults to "/tmp/".
#  CLEANUP: For REMOTE=true only.  If false, do not stop processes or delete
#    test files on remote hosts.  Defaults to true.
#  IMAGE_PROJECT: For REMOTE=true only.  Project containing images provided to
#  IMAGES.  Defaults to "kubernetes-node-e2e-images".
#  INSTANCE_PREFIX: For REMOTE=true only.  Instances created from images will
#    have the name "-".  Defaults to "test".
#  INSTANCE_METADATA: For REMOTE=true and running on GCE only.
#
# Example:
#   make test-e2e-node FOCUS=Kubelet SKIP=container
#   make test-e2e-node REMOTE=true DELETE_INSTANCES=true
#   make test-e2e-node TEST_ARGS="--cgroups-per-qos=true"
# Build and run tests.
---------------------------------------------------------------------------------
test-integration
# Build and run integration tests.
#
# Example:
#   make test-integration
---------------------------------------------------------------------------------
verify
# Runs all the presubmission verifications.
#
# Args:
#   BRANCH: Branch to be passed to verify-godeps.sh script.
#
# Example:
#   make verify
#   make verify BRANCH=branch_x
---------------------------------------------------------------------------------
vet
# Run 'go vet'.
#
# Args:
#   WHAT: Directory names to vet.  All *.go files under these
#     directories will be vetted.  If not specified, "everything" will be
#     vetted.
#
# Example:
#   make vet
#   make vet WHAT=pkg/kubelet
---------------------------------------------------------------------------------
```