powerpc/bootwrapper: Allow user to specify additional default targets

It is inconvenient to add additional default targets to the bootwrapper
Makefile for each new board supported which just needs a different dts
file.  This change allows the defconfig to specify additional build
targets.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
diff --git a/arch/powerpc/boot/Makefile b/arch/powerpc/boot/Makefile
index df6e621..88d30db 100644
--- a/arch/powerpc/boot/Makefile
+++ b/arch/powerpc/boot/Makefile
@@ -276,6 +276,9 @@
 image-$(CONFIG_PPC_PMAC)	+= zImage.coff zImage.miboot
 endif
 
+# Allow extra targets to be added to the defconfig
+image-y	+= $(subst ",,$(CONFIG_EXTRA_TARGETS))
+
 initrd-  := $(patsubst zImage%, zImage.initrd%, $(image-n) $(image-))
 initrd-y := $(patsubst zImage%, zImage.initrd%, \
 		$(patsubst dtbImage%, dtbImage.initrd%, \