Plugging ATF tests into the build and other cleanups

[Julio Merino]

Hello!

The next patch set is up for review. In this case, the goal is to plug
the ATF tests into the build.

Please note that the patches are ready for review but not ready for
submission. The built tests raise several errors and I still need to
diagnose if these are because of problems in my reachover Makefiles or
if they are just broken under my machine (powerpc64). That said,
fixing these should not affect the structure of the patches much, so
please go ahead and take a look!

While doing this, I have discovered a few minor issues that need
addressing and some improvements to be made, so I have tossed them
into the patches.

The one concern I have here is having to keep track of all tests in
tools/build/mk/OptionalObsoleteFiles.inc so that setting
WITHOUT_TESTS=no cleans up /usr/tests. This will be a pain to maintain
and a sure source of inconsistencies. If we could special-case this to
make it more automatic, do you have any suggestions?

As usual: http://portal.meroh.net/~jmmv/freebsd-testing/

Thanks!

-- 
Julio Merino / @jmmv