util/scripts/update_submodules: Fix "branch: unbound variable" error

After CB:86803, ${branch} variable is no longer valid. Use
${branch_name} instead ${branch} for generating the commit message.

TEST=./util/scripts/update_submodules -R 3rdparty/arm-trusted-firmware/
     The script generates the new commit successfully.

Change-Id: Ia528379b8721e6d419984bab28de7cf427e42423
Signed-off-by: Yidi Lin <yidilin@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/87268
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-by: Yu-Ping Wu <yupingso@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

util/scripts/update_submodules

@@ -132,7 +132,7 @@ 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##*/}
+	Update ${submodule##*/} submodule to upstream ${branch_name##*/}
 
 	Updating from commit id ${initial_commit_id}:
 	$initial_commit_description