Charlie Jacobsen authored Feb 17, 2016 and Vikram Narayanan committed Oct 26, 2016

I re-create the test_mods directory hierarchy in the build dir, and
bring over the Kbuilds. I also auto-generate the 'src =' line in
the Kbuilds for the external build hack.

Throughout the top-level Makefile, I use an additional hack. Let me
explain. We use recursive make to build dependencies (like versions
of libcap), because only the recursive make knows how to check that
the dependencies are up to date. But this means we have to make the
dependency targets phonies, so they always run. This in turn means
that those targets that depend on *them* will always run. To avoid
this, I use 'check for presence only' dependencies for these types
of dependencies that would trigger a full rebuild. The trick though
is to specify the final build output file as the dependency, rather
than the phony target, and to not create a rule for the final build
output file. (This way, if the file does not exist, the top-level
Makefile will just fail and say it doesn't know how to build the
file. It won't know that one of the phony targets does that.)

Anyway, test_mods is one example. It depends on liblcd.a being built,
but in order to prevent triggering a recursive make on liblcd.a, we
just use a 'check for presence only' type of dependency. It's up
to the person building to ensure they rebuild top-level targets
as necessary. (It doesn't seem easy to chain the whole build
together, with proper dependency management, since we're using
recursive make for dependency builds and kernel builds.)

build_all.sh updated to trigger build of all targets in the correct
order.