kconfig: show an error message when defconfig is not found

When a non-existing defconfig is specified,
display an easy-to-understand message
(fake the error message on Linux Kernel):

  $ make foo_defconfig
  ***
  *** Can't find default configuration "confis/foo_defconfig"!
  ***
Signed-off-by: NMasahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: NStephen Warren <swarren@nvidia.com>

scripts/multiconfig.sh

@@ -118,6 +118,13 @@ do_board_defconfig () {
 	defconfig_path=$srctree/configs/$1
 	tmp_defconfig_path=configs/.tmp_defconfig
 
+	if [ ! -r $defconfig_path ]; then
+		echo >&2 "***"
+		echo >&2 "*** Can't find default configuration \"confis/$1\"!"
+		echo >&2 "***"
+		exit 1
+	fi
+
 	mkdir -p arch configs
 	# defconfig for Normal:
 	#  pick lines without prefixes and lines starting '+' prefix