Create a new build.h on every make call; this makes sure it contains a

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>
Acked-by: Ronald G. Minnich <rminnich@gmail.com>
git-svn-id: svn://svn.coreboot.org/coreboot/trunk@5213 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1