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
[SQUASH ME] Fix some error + simplify some calls
make: Make BOARDS a global macro for buildtests
[SQUASH ME] make: info-boards-supported is just
make: Add supported boards to info-build target
[SQUASH ME] make: remove debugging lines
[SQUASH ME] make: Move buildtest related PHONY targets to Makefile.buildtests
Merge pull request #1557 from authmillenon/make-list-show

make: Add target to show if boards that are supported by apps