bootstd: USB devtype detection for script boot

Change the device type from "usb_mass_storage" to "usb" when
booting a script.

Before this change:
   => printenv devtype
   devtype=usb_mass_storage

After this change:
   => printenv devtype
   devtype=usb

Signed-off-by: John Clark <inindev@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/bootmeth_script.c b/boot/bootmeth_script.c
index 225eb18..a4050c3 100644
--- a/boot/bootmeth_script.c
+++ b/boot/bootmeth_script.c
@@ -190,7 +190,10 @@
 	ulong addr;
 	int ret;
 
-	ret = env_set("devtype", blk_get_devtype(bflow->blk));
+	if (desc->uclass_id == UCLASS_USB)
+		ret = env_set("devtype", "usb");
+	else
+		ret = env_set("devtype", blk_get_devtype(bflow->blk));
 	if (!ret)
 		ret = env_set_hex("devnum", desc->devnum);
 	if (!ret)