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/boot.c

@@ -57,6 +57,7 @@ int main(void)
 
 	led(0);
 
+	usb_reset();
 	run_payload();
 
 	while (1);	/* not reached */