x86: coreboot: Add support for Chromium OS verified boot

Add the required config and features to make this work. This creates a
new board called chromeos_coreboot.

Note that this board is not fully generic. The only board it currently
boots on is coral. It includes some coral-specific pieces in the
devicetree.

Update the documentation a little also.

Signed-off-by: Simon Glass <sjg@chromium.org>
Change-Id: I45ad9d40d507793b8565e60c345c5496eaa931be
12 files changed