efi_loader: correctly determine the boot partition

The device path of the loaded image should be set to the partition
from which the image was loaded. This requires using the same logic as
the load command.

Without the patch the device path pointed to the whole disk after executing

	load mmc 0: 0x43000000 FILE

and not to the boot partition from which the file was actually loaded.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>