kconfig: fix savedefconfig to output empty defconfig

Commit 3ff291f371
(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>

scripts/multiconfig.sh

@@ -217,6 +217,7 @@ do_savedefconfig () {
 	done
 
 	rm -f defconfig
+	touch defconfig
 	for line in $output_lines
 	do
 		echo $line >> defconfig