atusb/fw: implemented USB bus reset (to host) and polling of reset from host

- usb/usb.h, usb/atu2.c (usb_reset): reset the USB bus by detaching and
  re-attaching the device
- boot.c (main): force a USB reset before running the payload
- usb/atu2.c (usb_poll): test for USB reset from the host (in progress)
- usb/dfu.c (my_reset, dfu_init): register user USB reset handler

atusb/fw/usb/usb.h

@@ -141,6 +141,7 @@ void usb_io(struct ep_descr *ep, enum ep_state state, uint8_t *buf,
 
 int handle_setup(const struct setup_request *setup);
 int set_addr(uint8_t addr);
+void usb_reset(void);
 void usb_init(void);
 void usb_poll(void);