[POWERPC] bootwrapper: cuboot for 83xx

This adds cuboot support for MPC83xx platforms.

A device tree used with this must have linux,stdout-path in /chosen and
linux,network-index in any network device nodes that need mac addresses
assigned.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>