[POWERPC] prpmc2800 needs a dtbImage

The prpmc2800 platform requires a zImage formatted file with an
embedded dtb file.  Rename the requested boot image file to
dtbImage.prpmc2800.

Signed-off-by: Dale Farnsworth <dale@farnsworth.org>
Acked-by: Mark A. Greer <mgreer@mvista.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Paul Mackerras <paulus@samba.org>