bootstd: Add a new pre-scan priority for bootdevs

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>

doc/develop/bootstd.rst

@@ -211,7 +211,7 @@ A bootdev driver is typically fairly simple. Here is one for mmc::
     {
         struct bootdev_uc_plat *ucp = dev_get_uclass_plat(dev);
 
-        ucp->prio = BOOTDEVP_0_INTERNAL_FAST;
+        ucp->prio = BOOTDEVP_2_INTERNAL_FAST;
 
         return 0;
     }