This allows using only one of either raw or fs mode for SPL mmc boot, without
the need to have provisions for the other. In particular, a device may have
U-Boot installed on a file system on the mmc, without ever needing to read
U-Boot from raw memory. Thus, there is no reason to provide a sector or
partition for raw mode. This allows this behaviour and still provides a robust
fallback mechanism in case provisions for both modes are defined.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>