bootstd: Add a USB hunter

Add a hunter for USB which enumerates the bus to find new bootdevs.

Update the tests and speed up bootdev_test_prio() while we are here, by
dropping the USB delays.

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

drivers/usb/host/usb_bootdev.c

@@ -20,6 +20,11 @@ static int usb_bootdev_bind(struct udevice *dev)
 	return 0;
 }
 
+static int usb_bootdev_hunt(struct bootdev_hunter *info, bool show)
+{
+	return usb_init();
+}
+
 struct bootdev_ops usb_bootdev_ops = {
 };
 
@@ -35,3 +40,10 @@ U_BOOT_DRIVER(usb_bootdev) = {
 	.bind		= usb_bootdev_bind,
 	.of_match	= usb_bootdev_ids,
 };
+
+BOOTDEV_HUNTER(usb_bootdev_hunter) = {
+	.prio		= BOOTDEVP_3_SCAN_SLOW,
+	.uclass		= UCLASS_USB,
+	.hunt		= usb_bootdev_hunt,
+	.drv		= DM_DRIVER_REF(usb_bootdev),
+};