Add preprocessing capabilities to the cbfs-files mechanism

Add preprocessing capabilities to the cbfs-files mechanism

It's now possible to generate files that are about to be added to
CBFS by specifying "sourcefile:method" as real file name.

This makes the build system use the cbfs-files-preprocessor-$(method)
function to create a file from sourcefile. That generated file is
then added to CBFS.

The first method to be defined is "nvramtool". It expects a plain text
specification of the CMOS configuration and emits the binary format
suitable for cmos.default.

Change-Id: I33a142718fc7238eaf5317b0ed62b4726d9b48f2
Signed-off-by: Patrick Georgi <Patrick.Georgi@secunet.com>
Reviewed-on: http://review.coreboot.org/847
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>