kbuild: allow empty board directories
U-Boot has compelled all boards to have
board/${BOARD}/ or board/${VENDOR}/${BOARD}/ directory.
Sometimes it does not seem suitable for some boards,
for example Sandbox. (Is it a board?)
And arcangel4 board has nothing to compile
under the board directory.
This commit makes the build system more flexible:
If '<none>' is given to the 6th column (=Board name) of boards.cfg,
Kbuild will not descend into the board directory.
Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>