[POWERPC] bootwrapper: Add 8xx cuboot support

This allows booting on legacy, non-device-tree aware versions of U-boot.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>