[S390] set KBUILD_IMAGE.

Set KBUILD_IMAGE to a sane value. This enables "make rpm"
Signed-off-by: Christian Borntraeger <cborntra@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/Makefile

@@ -35,6 +35,9 @@ cflags-$(CONFIG_MARCH_Z900) += $(call cc-option,-march=z900)
 cflags-$(CONFIG_MARCH_Z990) += $(call cc-option,-march=z990)
 cflags-$(CONFIG_MARCH_Z9_109) += $(call cc-option,-march=z9-109)
 
+#KBUILD_IMAGE is necessary for make rpm
+KBUILD_IMAGE	:=arch/s390/boot/image
+
 #
 # Prevent tail-call optimizations, to get clearer backtraces:
 #