[POWERPC] bootwrapper: Update .gitignore

All cuImage types are ignored, as well as preprocessed .lds files,
and the forthcoming zImage.bin files and embedded planet board images.

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>