bootstd: Keep track of use of usb stop

When 'usb stop' is run, doing 'bootflow scan' does not run the USB hunter
again so does not see any devices. Fix this by telling bootstd about the
state of USB.

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

include/bootdev.h

@@ -320,6 +320,15 @@ int bootdev_hunt(const char *spec, bool show);
  */
 int bootdev_hunt_prio(enum bootdev_prio_t prio, bool show);
 
+/**
+ * bootdev_unhunt() - Mark a device as needing to be hunted again
+ *
+ * @id: uclass ID to update
+ * Return: 0 if done, -EALREADY if already in this state, -ENOENT if no hunter
+ * found for that uclass
+ */
+int bootdev_unhunt(enum uclass_id id);
+
 /**
  * bootdev_hunt_and_find_by_label() - Hunt for bootdevs by label
  *