boards.cfg: change "<none>" in the board field to "-"

In the previous commit, all the board fields were filled.

Now we can use "-" in the board field for a different meaning.

Going forward, "-" stands for no board directory
as in cpu, soc, vendor fields.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/mkconfig b/mkconfig
index 401f262..84f5a77 100755
--- a/mkconfig
+++ b/mkconfig
@@ -60,13 +60,7 @@
 	cpu=
 fi
 
-if [ "$6" = "<none>" ] ; then
-	board=
-elif [ "$6" = "-" ] ; then
-	board=${BOARD_NAME}
-else
-	board="$6"
-fi
+[ "$6" != "-" ] && board="$6"
 [ "$5" != "-" ] && vendor="$5"
 [ "$4" != "-" ] && soc="$4"
 [ $# -gt 7 ] && [ "$8" != "-" ] && {