update_submodules: Prefix commit title with relative path

It's common for coreboot commits to prefix the commit title with a
topic, which is often the path to the files or directories. So adjust
this commit title accordingly.

Change-Id: Ice267719a08b289b0d996fd20c993d616c812d00
Signed-off-by: Felix Singer <felixsinger@posteo.net>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/87824
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-by: Matt DeVillier <matt.devillier@gmail.com>

util/scripts/update_submodules

@@ -136,7 +136,7 @@ main() {
 			cd "${TOP}" || exit 1
 			git add "${submodule}" > /dev/null 2>&1 || exit 1
 			git commit -s -F- > /dev/null 2>&1 <<-EOF
-	Update ${submodule##*/} submodule to upstream ${branch_name##*/}
+	${submodule}: Update to upstream ${branch_name##*/}
 
 	Updating from commit id ${initial_commit_id}:
 	$initial_commit_description