cmake: check BOARD and SOC roots before use

People frequently do things like setting BOARD_ROOT to
/my/module/boards/arm/my_board when it should be set to /my/module.

Similar misconfigurations happen for SOC_ROOT.

Emit warnings in these situations to try to point users in the right
direction.

A similar change could be made for DTS_ROOT, but that is trickier
since at least one directory (the application root) is legitimately
added to it that does not always contain dts/ or include/
subdirectories.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>