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>

config.mk

@@ -28,11 +28,13 @@ sinclude $(TOPDIR)/$(CPUDIR)/config.mk		# include  CPU	specific rules
 ifdef	SOC
 sinclude $(TOPDIR)/$(CPUDIR)/$(SOC)/config.mk	# include  SoC	specific rules
 endif
+ifneq ($(BOARD),)
 ifdef	VENDOR
 BOARDDIR = $(VENDOR)/$(BOARD)
 else
 BOARDDIR = $(BOARD)
 endif
+endif
 ifdef	BOARD
 sinclude $(TOPDIR)/board/$(BOARDDIR)/config.mk	# include board specific rules
 endif