arch/x86: avoid race condition on build.h

Change-Id: I15375ac1247b7cc8d80d910a767c7f3e67eb8739
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: http://review.coreboot.org/11904
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Nico Huber <nico.h@gmx.de>

src/arch/x86/Makefile.inc

@@ -85,6 +85,8 @@ endif
 # the right order. Make sure the auto generated bootblock.inc is a proper
 # dependency. Make the same true for the linker sript.
 bootblock-y += id.S
+$(obj)/arch/x86/id.bootblock.o: $(obj)/build.h
+
 bootblock-y += bootblock.S
 bootblock-y += walkcbfs.S
 $(obj)/arch/x86/bootblock.bootblock.o: $(objgenerated)/bootblock.inc