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>

Coreboot tables provide information about the CMOS-RAM checksum. Add a
command which can check and update this.

With this it is possible to adjust CMOS-RAM settings and tidy up the
checksum afterwards.

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

Update the cbsysinfo command to show the contents of the CMOS option
table.

While we are here, add some example output for this command, along with
mention of what the unimplemented tags are.

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

Add a simple test for this command, checking that coreboot has the
required features.

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