rom.sh: also add grub to seabios images

we want multiple seagrub images made, with different
keymaps, but we only want one non-seagrub image.

however, we also want grub in the non-seagrub image.
it just means that seabios is primarily what the user
wants, and they might occasionally use grub, whereas
the seagrub images are for people who primarily want
grub but may occasionally access the seabios menu.

right now, the seabios images really only contain seabios,
but there's no harm in adding grub to them.

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