[POWERPC] bootwrapper: Only build cuImage if CONFIG_DEVICE_TREE is non-empty

This allows the zImage target to once again be used to build
all supported image types, rather than requiring an explicit
"make uImage" to avoid failing to create an unneeded cuImage.

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>