Add documentation for the sandbox bootmeth.
Fix up the compatible string to drop the 'extlinux' part, which is not
relevant to this bootmeth.
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
