[POWERPC] Small cleanups to the cuboot bootwrapper code

This patch makes a few small cleanups to the cuboot code.
	- It removes the double layered selection of images, via
cuboot-plat-y, instead having the cuboot platforms directly select a
suitable image-y (this changes the name of the final cuboot image from
plain cuImage to cuImage.<platform>).
	- Factors out some code in the wrapper that's potentially
useful to platforms other than uboot.

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