Make $(obj)/build.h an explicit dependency for all

Make $(obj)/build.h an explicit dependency for all
object files, as it's dragged into the build by
-include $(obj)/build.h for pretty much everything.

Also, generate build.h atomically.

This fixes parallel build with kconfig on my box.

Signed-off-by: Patrick Georgi <patrick.georgi@coresystems.de>
Acked-by: Ronald G. Minnich <rminnich@gmail.com>
git-svn-id: svn://svn.coreboot.org/coreboot/trunk@5062 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1