powerpc: Use scripts/mkuboot.sh instead of 'mkimage'

mkuboot.sh provides a basic wrapper for the 'mkimage' utility.  Using
mkuboot.sh provides clearer error reporting and allows a toolchain to
use its own 'mkimage' executable specified by ${CROSS_COMPILE}mkimage.
Additionally, this brings PowerPC in line with other architectures
which already call mkimage via mkuboot.sh.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>