[ARM] Add KBUILD_IMAGE target support

Add support for KBUILD_IMAGE on ARM.  This takes the usual target
specifiers (zImage/Image/etc) in the same way that powerpc does
(iow, without the arch/arm/boot prefix).
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>

arch/arm/Makefile

@@ -174,11 +174,13 @@ libs-y				:= arch/arm/lib/ $(libs-y)
 
 # Default target when executing plain make
 ifeq ($(CONFIG_XIP_KERNEL),y)
-all: xipImage
+KBUILD_IMAGE := xipImage
 else
-all: zImage
+KBUILD_IMAGE := zImage
 endif
 
+all:	$(KBUILD_IMAGE)
+
 boot := arch/arm/boot
 
 #	Update machine arch and proc symlinks if something which affects