ARM: Fix build error when using KCONFIG_CONFIG

Jonathan Cameron reports that when using the environment
variable KCONFIG_CONFIG, he encounters this error:

make[2]: *** No rule to make target `.config', needed by `arch/arm/boot/compressed/vmlinux.lds'
Reported-by: NJonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>

arch/arm/boot/compressed/Makefile

@@ -116,5 +116,5 @@ CFLAGS_font.o := -Dstatic=
 $(obj)/font.c: $(FONTC)
 	$(call cmd,shipped)
 
-$(obj)/vmlinux.lds: $(obj)/vmlinux.lds.in arch/arm/boot/Makefile .config
+$(obj)/vmlinux.lds: $(obj)/vmlinux.lds.in arch/arm/boot/Makefile $(KCONFIG_CONFIG)
 	@sed "$(SEDFLAGS)" < $< > $@