[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: NDale Farnsworth <dale@farnsworth.org>
Acked-by: NMark A. Greer <mgreer@mvista.com>
Acked-by: NGrant Likely <grant.likely@secretlab.ca>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

arch/powerpc/boot/Makefile

@@ -194,7 +194,7 @@ image-$(CONFIG_PPC_CHRP)		+= zImage.chrp
 image-$(CONFIG_PPC_EFIKA)		+= zImage.chrp
 image-$(CONFIG_PPC_PMAC)		+= zImage.pmac
 image-$(CONFIG_PPC_HOLLY)		+= zImage.holly
-image-$(CONFIG_PPC_PRPMC2800)		+= zImage.prpmc2800
+image-$(CONFIG_PPC_PRPMC2800)		+= dtbImage.prpmc2800
 image-$(CONFIG_PPC_ISERIES)		+= zImage.iseries
 image-$(CONFIG_DEFAULT_UIMAGE)		+= uImage