tests: let license checker check headers, too
make: Add target to show if boards that are supported by apps
[SQUASH ME] make: move info-boards-supported to Makefile.buildtests
make: Add info prefix for non-building targets in Makefile.buildtests
[SQUASH ME] make: Add info prefix for non-building targets in Makefile.buildtests
changed parameters form `kernel_pid_t` to `int`
pthread: pthread_barrier should call yield
tests: clean up test_pthread_barrier Makefile
[SQUASH ME] Fix some error + simplify some calls
make: don't call mkdir -p a lot
make: Make BOARDS a global macro for buildtests
[SQUASH ME] make: info-boards-supported is just
make: Add supported boards to info-build target
make: use $@ and $<
make: call ranlib
make: don't invoke wildcard repeatedly
make: easier to read object targets
[SQUASH ME] make: remove debugging lines
sys: remove lib/hashtable.c

Quoting myself:

> The code of sys/lib/hashtable.c is very badly adapted for embedded
> systems. The used primes are huge, and the limit is entirely
> unrealistic. Why do we ship this file?

In #1564 we came to the conclusion that we don't need this code.

Closes #1564.
[SQUASH ME] make: Move buildtest related PHONY targets to Makefile.buildtests