Add support for VPL, a new phase of U-Boot. This runs after TPL. It is responsible for selecting which SPL binary to run, based on a verified-boot process. Signed-off-by: Simon Glass <sjg@chromium.org>
Add a bit more information about how to use SPL. Signed-off-by: Simon Glass <sjg@chromium.org>
Move this documentation over to .rst format. Signed-off-by: Simon Glass <sjg@chromium.org>