kconfig: config script: add a little user help

Give the user a clue about the problem along with the 35 lines of
usage/help text.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: linux-kbuild@vger.kernel.org
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/config

@@ -223,6 +223,7 @@ while [ "$1" != "" ] ; do
 		;;
 
 	*)
+		echo "bad command: $CMD" >&2
 		usage
 		;;
 	esac