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: NGrant Likely <grant.likely@secretlab.ca>
Acked-by: NJosh Boyer <jwboyer@linux.vnet.ibm.com>

arch/powerpc/Kconfig

@@ -458,6 +458,19 @@ config CMDLINE
 	  some command-line options at build time by entering them here.  In
 	  most cases you will need to specify the root device here.
 
+config EXTRA_TARGETS
+	string "Additional default image types"
+	help
+	  List additional targets to be built by the bootwrapper here (separated
+	  by spaces).  This is useful for targets that depend of device tree
+	  files in the .dts directory.
+
+	  Targets in this list will be build as part of the default build
+	  target, or when the user does a 'make zImage' or a
+	  'make zImage.initrd'.
+
+	  If unsure, leave blank
+
 if !44x || BROKEN
 config ARCH_WANTS_FREEZER_CONTROL
 	def_bool y

arch/powerpc/boot/Makefile

@@ -276,6 +276,9 @@ ifeq ($(CONFIG_PPC32),y)
 image-$(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%, \