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>
diff --git a/drivers/scsi/scsi_bootdev.c b/drivers/scsi/scsi_bootdev.c
index 2367b33..991013f 100644
--- a/drivers/scsi/scsi_bootdev.c
+++ b/drivers/scsi/scsi_bootdev.c
@@ -16,7 +16,7 @@
 {
 	struct bootdev_uc_plat *ucp = dev_get_uclass_plat(dev);
 
-	ucp->prio = BOOTDEVP_2_SCAN_FAST;
+	ucp->prio = BOOTDEVP_4_SCAN_FAST;
 
 	return 0;
 }
@@ -55,7 +55,7 @@
 };
 
 BOOTDEV_HUNTER(scsi_bootdev_hunter) = {
-	.prio		= BOOTDEVP_2_SCAN_FAST,
+	.prio		= BOOTDEVP_4_SCAN_FAST,
 	.uclass		= UCLASS_SCSI,
 	.hunt		= scsi_bootdev_hunt,
 	.drv		= DM_DRIVER_REF(scsi_bootdev),