create a new build.h on every make call; this makes sure it contains a valid
compiler signature and time stamp. Since we maintain correct build.h
dependencies in the source code we can also drop "prepare2"

Signed-off-by: Stefan Reinauer <stepan@coresystems.de>

Index: Makefile
===================================================================
--- Makefile	(revision 5207)
+++ Makefile	(working copy)
@@ -275,7 +275,7 @@
 CBFS_PAYLOAD_COMPRESS_FLAG:=l
 endif
 
-coreboot: prepare prepare2 $(obj)/coreboot.rom
+coreboot: prepare $(obj)/coreboot.rom
 
 endif
 
@@ -284,9 +284,7 @@
 	mkdir -p $(obj)/util/kconfig/lxdialog $(obj)/util/cbfstool
 	test -n "$(alldirs)" && mkdir -p $(alldirs) || true
 
-prepare2: $(obj)/build.h
-
-$(obj)/build.h:
+$(obj)/build.h: .xcompile
 	@printf "    GEN        build.h\n"
 	rm -f $(obj)/build.h
 	printf "/* build system definitions (autogenerated) */\n" > $(obj)/build.ht
@@ -355,5 +353,5 @@
 	@# http://www.coreboot.org/pipermail/coreboot/2010-February/055825.html
 	$(HOSTCC) -g $(STACK) -Wall -o $@ $<
 
-.PHONY: $(PHONY) prepare prepare2 clean distclean doxygen doxy coreboot
+.PHONY: $(PHONY) prepare clean distclean doxygen doxy coreboot .xcompile