Sometimes REPORT=1 just wasn't passed to make, and the build.log
saved is noisy and harder to compare through diff(1); let's move
the implementation out of the makefile and into a standalone
script so it can be reused like this:

  BUILDDIR=$(sed -rn "s/^mki-image-scripts: export GLOBAL_BUILDDIR='(.*)'$/\1/p" build/build.log
  bin/cleanlog < build/build.log > build/reports/clean.log

See "export GLOBAL_BUILDDIR=" line in the particular log
(grabbing that one from stdin is not exactly trivial though).