git.sh: allow patching submodules

for single-tree project (e.g. flashprog):
config/submodule/PROJECT/MODNAME/patches

for multi-tree project (e.g. coreboot):
config/submodule/PROJECT/TREE/MODNAME/patches

MODNAME is e.g.:
3rdparty/vboot directory in coreboot: would become vboot
(the submodule codepath is filtered to up to the final slash)

another example:
submodire src dir 3rdparty/foo/bar
MODNAME would be "bar"

Add whatever patches you like to a given submodule.

An example patch is included in this commit.

Signed-off-by: Leah Rowe <leah@libreboot.org>

config/submodule/coreboot/default/vboot/patches/0001-test-patch.patch

@@ -0,0 +1,26 @@
+From 68f99140dda455a4f4e9d73df6d7b0a504494e95 Mon Sep 17 00:00:00 2001
+From: Leah Rowe <info@minifree.org>
+Date: Sun, 19 May 2024 23:35:52 +0100
+Subject: [PATCH 1/1] test patch
+
+Signed-off-by: Leah Rowe <info@minifree.org>
+---
+ Makefile | 2 ++
+ 1 file changed, 2 insertions(+)
+
+diff --git a/Makefile b/Makefile
+index 4cb265b2..ef29489f 100644
+--- a/Makefile
++++ b/Makefile
+@@ -2,6 +2,8 @@
+ # Use of this source code is governed by a BSD-style license that can be
+ # found in the LICENSE file.
+ 
++# test
++
+ # This Makefile normally builds in a 'build' subdir, but use
+ #
+ #    make BUILD=<dir>
+-- 
+2.39.2
+