Add nvramtool to coreboot build system

Add nvramtool to coreboot build system

This way we can depend on it during build.

Change-Id: I7e773c6a029e376e3d70d0a8c9e96ffe0c2cf82e
Signed-off-by: Patrick Georgi <Patrick.Georgi@secunet.com>
Reviewed-on: http://review.coreboot.org/845
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>