[POWERPC] Clean images in arch/powerpc/boot

Add a rule to clean up the various generated image files in
arch/powerpc/boot.
Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/boot/Makefile

@@ -179,3 +179,4 @@ install: $(CONFIGURE) $(image-y)
 
 clean-files += $(addprefix $(objtree)/, $(obj-boot) vmlinux.strip.gz)
 clean-files += $(addprefix $(objtree)/, $(obj-boot) vmlinux.bin.gz)
+clean-files += $(image-)