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

This way it will be easier to check ahead, if a board is not supported by an application
Using `$(RIOTBOARD)` is the crucial part, or it won't work for 3rd party boards.
And I guess in Makefile.buildtests:33 and in here we need use the intersection between the white listed boards and the available boards, also because of 3rd party boards.