doc: Update documentation for cros-2021.04 release

With the new 2021.04 we have a new version of Chromium OS boot, which
supports sandbox, coral and coral-on-coreboot. Add documentation for
this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
1 file changed