use/repo/main for vm targets will now create and enable the main
repository in /ALTLinux on rootfs.

This reverts commit 1b457a5d.
It wasn't prudent to switch everyone to master FTP server;
Yandex mirror still has an order of magnitude more bandwidth.

%_host_cpu was a wrong choice in the first place,
and the particular case of %_arch == ppc64le
with %_host_cpu == powerpc64le became the culprit.

Yandex mirror can lag behind at times:
http://mirmon.altlinux.org/sisyphus.html#ru

...and we've got 1GbE there now, not that bad
compared to 100Mbps back then...

NB: yandex traffic might be billed as "local"
    in many regions, this should be revisited.

This is to make building addon images possible;
those carrying packages but lacking any stage2.

...instead of (often-crowded) ftp.altlinux.org for updates.

Those few places lacking "; @:" replacement for a real recipe
have been finally -spotted- identified with `make check'
and fixed.

rpm-4.13 stopped bringing alt-gpgkeys in, and specifying it
explicitly for STAGE1_PACKAGES didn't help somehow; don't presume
/usr/lib/alt-gpgkeys/ existence anyways.

Old format:
rpm [p7] http://ftp.altlinux.org/pub/distributions/ALTLinux/p7/branch x86_64 classic
New format:
rpm [p8] http://ftp.altlinux.org/pub/distributions/ALTLinux p8/branch/x86_64 classic

Installation finishes just fine if bzip2-compressed
hash files are removed *but* apt-cdrom add will fail.

This is about (a)synchronous package builder in fact.

See http://www.opennet.ru/openforum/vsluhforumID3/86239.html#1
for a query that has led to this one; in particular,
- xdm dropped (won't log in root and there are no users yet);
- network is brought up and configured via DHCP by default;
- apt-get works out-of-box;
- default image size is twice the chroot size.

This further refines the modular build by making
metadata being a clearly separated feature rather
than having to rely on runtime tests, and also by
moving the code which cares for kernel bits of base
installation (.base list) in a feature of its own.

There's more to it but let's get the ball rolling first.

Just a hook to export alt-gpgkeys pubring data.

The features might get copy-pasted (or even copied-and-pruned)
when initialized; there's an unneccessary duplication of the
function name in the line adding it to FEATURES list, thus
prone to being forgotten and causing some havoc later on.

It was wrong in the first place but tackling this with some
double-colon rules ran into terminality issues, and further
tortures were considered unneccessary.

The current solution isn't perfect (no completely transparent
function name registration upon corresponding target being called)
but at least it is an improvement...

We've got some parts of it in build-distro feature,
and some went to dev feature for no real reason.

But a bare installer might go without package base,
and LiveCDs other than live-builder might find local
repository useful given aufs2 root overlay.

Now the overall scheme is more straightforward:
- a distro:
  + asks that a package repo be included
  + cares to further add the packages to it
- a repo feature:
  + pulls in sub/main for it to happen
  + provides genbasedir script to create repo metadata
  + supplements live feature with repo configuration