Undo f695681 (r30433). USB hardware must be initialized first.

This fixes flash/ MMC access on the Ondios when booting from ROM.
Add a comment what to pay attention for in the target specific
usb_init_device().

firmware/usb.c

@@ -659,6 +659,11 @@ void usb_acknowledge(long id)
 
 void usb_init(void)
 {
+    /* Do required hardware inits first. For software USB the driver has
+     * to make sure this won't trigger a transfer completion before the
+     * queue and thread are created. */
+    usb_init_device();
+
 #ifdef USB_FULL_INIT
     usb_enable(false);
 
@@ -672,8 +677,6 @@ void usb_init(void)
     tick_add_task(usb_tick);
 #endif
 #endif /* USB_FULL_INIT */
-
-    usb_init_device();
 }
 
 void usb_wait_for_disconnect(struct event_queue *q)