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/usage/cmd/cbcmos.rst

@@ -40,3 +40,6 @@ CMOS RAM::
     Checksum 6600 written
     => cbc check
     =>
+
+See also :ref:`cedit_cb_load` which shows an example that includes the
+configuration editor.