Fix coreboot updates

The rule to prepare a new coreboot.pre1 was ignored in the
"update image" scenario because a perfectly fine file exists.
Mark it phony to fix it.

Change-Id: Ie7f8b36b71015a593958cd6e19602bad6b854320
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/351
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/arch/x86/Makefile.inc

@@ -66,6 +66,7 @@ $(obj)/coreboot.pre1: $(obj)/coreboot.bootblock $$(prebuilt-files) $(CBFSTOOL)
 	$(CBFSTOOL) $@ create $(CONFIG_COREBOOT_ROMSIZE_KB)K $(obj)/coreboot.bootblock
 	$(prebuild-files)
 else
+.PHONY: $(obj)/coreboot.pre1
 $(obj)/coreboot.pre1: $(CBFSTOOL)
 	mv $(obj)/coreboot.rom $@
 endif