tinderbox: return the exit status of the make function override.

This ensure that tests are correctly detected as passed/failed by
portage.
Also, calling /usr/bin/make instead of emake should more
accurately replicate the behaviour intended by the original
function call.
tinderbox: remove references to pkgcore-checks, repoman, and DESCRIPTION.

These will now be collected at a different stage of the process.
tinderbox: change default storage directory.

Collected data is intended to persist unless manually removed, so
/var/lib is a better fit as per FHS.
tinderbox: improve comments.
depcheck: add script for checking binary runtime dependencies.