re-add ability to use cbfs grub.cfg as default

i removed this before, when making grub multi-tree,
because the design i used in an earlier version of
the patch actually added the grub.elf generation
to grub source itself, but then i decided to hack
around the grub build system from lbmk/cbmk instead

re-add this functionality, so that users can easily
insert their own custom grub.cfg into cbfs without
needing to re-build their image.

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

config/data/grub/memdisk.cfg

@@ -0,0 +1,10 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+# Copyright (C) 2023 Leah Rowe <leah@libreboot.org>
+
+set prefix=(memdisk)/boot/grub
+
+if [ -f (cbfsdisk)/grub.cfg ]; then
+	source (cbfsdisk)/grub.cfg
+else
+	source (memdisk)/boot/grub/grub_default.cfg
+fi