grub-efi: Allow to use file name different from grub.cfg in GRUBCFG
To generate a same image both in live image type and boot-directdisk image type
and make boot-direct image boot directly on the rootfs partition without using
an initramfs we need to have the ability to generate several grub.cfg files.
Signed-off-by: Nicolas Guyomard <nicolas.guyomard@open.eurogiciel.org> Signed-off-by: Ross Burton <ross.burton@intel.com>