x86: Add chromebook_link board

This board is a 'bare' version of the existing 'link 'board. It does not
require coreboot to run, but is intended to start directly from the reset
vector.

This initial commit has place holders for a wide range of features. These
will be added in follow-on patches and series. So far it cannot be booted
as there is no ROM image produced, but it does build without errors.

Signed-off-by: Simon Glass <sjg@chromium.org>