util/grub-mkrescue: Search by file UUID rather than partition UUID for EFI boot

util/grub-mkrescue: Search by file UUID rather than partition UUID for EFI boot

The final piece needed to add UEFI file system transposition support is to
ensure the boot media can be located regardless of how the boot partition
was instantiated. Especially, we do not want to be reliant on brittle
partition UUIDs, as these only work if a boot media is duplicated at the
block level and not at the file system level.

To accomplish this for EFI boot, we now create a UUID file in a .disk/
directory, that can then be searched for.

Note: The switch from make_image_fwdisk_abs() to make_image_abs() is
needed in order to use the search functionality.

Signed-off-by: Pete Batard <pete@akeo.ie>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>