kconfig: merge_config: use an empty file as initfile

The scripts/kconfig/merge_config.sh script requires an existing
$INITFILE (or the $1 argument) as a base file for merging Kconfig
fragments. However, an empty $INITFILE can serve as an initial starting
point, later referenced by the KCONFIG_ALLCONFIG Makefile variable
if -m is not used. This variable can point to any configuration file
containing preset config symbols (the merged output) as stated in
Documentation/kbuild/kconfig.rst. When -m is used $INITFILE will
contain just the merge output requiring the user to run make (i.e.
KCONFIG_ALLCONFIG=<$INITFILE> make <allnoconfig/alldefconfig> or make
olddefconfig).

Instead of failing when `$INITFILE` is missing, create an empty file and
use it as the starting point for merges.

Signed-off-by: Daniel Gomez <[e-mail]>
Signed-off-by: Masahiro Yamada <[e-mail]>

scripts/kconfig/merge_config.sh

···
 shift;
 
 if [ ! -r "$INITFILE" ]; then
-	echo "The base file '$INITFILE' does not exist.  Exit." >&2
-	exit 1
+	echo "The base file '$INITFILE' does not exist. Creating one..." >&2
+	touch "$INITFILE"
 fi
 
 MERGE_LIST=$*