kconfig: fix savedefconfig to output empty defconfig

Commit 3ff291f371fa9858426774f3732924bacb61ed1c
(kconfig: convert Kconfig helper script into a shell script)
introduced a minor regression.

make alldefconfig; make savedefconfig
should create an empty 'defconfig'.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/scripts/multiconfig.sh b/scripts/multiconfig.sh
index 4190798..785f563 100644
--- a/scripts/multiconfig.sh
+++ b/scripts/multiconfig.sh
@@ -217,6 +217,7 @@
 	done
 
 	rm -f defconfig
+	touch defconfig
 	for line in $output_lines
 	do
 		echo $line >> defconfig