[POWERPC] cell: fix default zImage build target

Change CONFIG_PPC_CELL to CONFIG_PPC_IBM_CELL_BLADE in the powerpc boot
makefile.

CONFIG_PPC_CELL is used to build the generic cell processor support, and
is not an indication of platform.
Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/boot/Makefile

@@ -153,7 +153,7 @@ $(obj)/uImage: vmlinux $(wrapperbits)
 
 image-$(CONFIG_PPC_PSERIES)		+= zImage.pseries
 image-$(CONFIG_PPC_MAPLE)		+= zImage.pseries
-image-$(CONFIG_PPC_CELL)	+= zImage.pseries
+image-$(CONFIG_PPC_IBM_CELL_BLADE)	+= zImage.pseries
 image-$(CONFIG_PPC_CHRP)		+= zImage.chrp
 image-$(CONFIG_PPC_PMAC)		+= zImage.pmac
 image-$(CONFIG_DEFAULT_UIMAGE)		+= uImage