x86: coreboot: Allow building an expo for editing CMOS config

Coreboot provides the CMOS layout in the tables it passes to U-Boot.
Use that to build an editor for the CMOS settings.

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

doc/develop/cedit.rst

@@ -172,4 +172,4 @@ Cedit provides several options for persistent settings:
 
 For now, reading and writing settings is not automatic. See the
 :doc:`../usage/cmd/cedit` for how to do this on the command line or in a
-script.
+script. For x86 devices, see :ref:`cedit_cb_load`.