[POWERPC] bootwrapper: add simpleImage* to list of boot targets

Without simpleImage% in the BOOT_TARGETS list, it is impossible to
build any of the simpleImages.
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>

arch/powerpc/Makefile

@@ -154,7 +154,7 @@ all: zImage
 
 CPPFLAGS_vmlinux.lds	:= -Upowerpc
 
-BOOT_TARGETS = zImage zImage.initrd uImage zImage% dtbImage% treeImage.% cuImage.%
+BOOT_TARGETS = zImage zImage.initrd uImage zImage% dtbImage% treeImage.% cuImage.% simpleImage.%
 
 PHONY += $(BOOT_TARGETS)