It seems better to call this a 'bootdev' since this is name used in the
documentation. The older 'Bootdevice' name is no-longer used and may cause
confusion with the 'bootdevice' environment variable.

Update throughout to use bootdev.

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

We need extensions to be set up before we start trying to boot any of the
bootdevs. Add a new priority before all the others for tht sort of thing.
Also add a 'none' option, so that the first one is not 0.

While we are here, comment enum bootdev_prio_t fully and expand the test
for the 'bootdev hunt' command.

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

Add a bootdev for SCSI so that these devices can be used with standard
boot.

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