Add support for cbfs-files-y to the build system.

Add support for cbfs-files-y to the build system.
That variable allows chipset components to add files to
the CBFS image, for details see
http://www.coreboot.org/pipermail/coreboot/2010-December/062483.html

Compared to the patch in that mail this commit improves dependency
tracking a bit.

Signed-off-by: Patrick Georgi <patrick.georgi@coresystems.de>
Acked-by: Joseph Smith <joe@settoplinux.org>
Acked-by: Stefan Reinauer <stepan@coreboot.org>
git-svn-id: svn://svn.coreboot.org/coreboot/trunk@6182 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1