Revert "staging: board: Remove macro board_staging"

This reverts commit 850c35bb as it
breaks the build.

Cc: Song Chen <chensong_2000@189.cn>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/20210104122653.6f35b9bb@canb.auug.org.au


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>