spl: nand: support loading legacy image with payload compressed

Add support to load legacy image with payload compressed. This redirects
the boot flow for all legacy images. If the payload is not compressed, the
actual behavior will remain unchanged.

Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>