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>
diff --git a/mkconfig b/mkconfig
index 5f516f2..9827e4d 100755
--- a/mkconfig
+++ b/mkconfig
@@ -55,7 +55,9 @@
 arch="$2"
 cpu=`echo $3 | awk 'BEGIN {FS = ":"} ; {print $1}'`
 spl_cpu=`echo $3 | awk 'BEGIN {FS = ":"} ; {print $2}'`
-if [ "$6" = "-" ] ; then
+if [ "$6" = "<none>" ] ; then
+	board=
+elif [ "$6" = "-" ] ; then
 	board=${BOARD_NAME}
 else
 	board="$6"
@@ -177,8 +179,8 @@
 
 [ "${soc}"    ] && echo "#define CONFIG_SYS_SOC    \"${soc}\""    >> config.h
 
+[ "${board}"  ] && echo "#define CONFIG_BOARDDIR board/$BOARDDIR" >> config.h
 cat << EOF >> config.h
-#define CONFIG_BOARDDIR board/$BOARDDIR
 #include <config_cmd_defaults.h>
 #include <config_defaults.h>
 #include <configs/${CONFIG_NAME}.h>