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>
070aee6728