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

script/trees

@@ -284,7 +284,9 @@ mkpayload_grub()
 	    -O i386-coreboot -o "${cdir}/grub.elf" -d "${cdir}/grub-core/" \
 	    --fonts= --themes= --locales=  --modules="$grub_modules" \
 	    --install-modules="$grub_install_modules" \
-	    "/boot/grub/grub.cfg=${cdir}/.config" || $err "$tree: !mkgrub"
+	    "/boot/grub/grub_default.cfg=${cdir}/.config" \
+	    "/boot/grub/grub.cfg=$grubdata/memdisk.cfg" || \
+	    $err "$tree: cannot build grub.elf"
 }
 
 copy_elf()