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/atu2.c

@@ -171,6 +171,12 @@ void usb_poll(void)
 {
 	uint8_t flags, i;
 
+	flags = UDINT;
+	if (flags & EORSTI) {
+		UDINT &= ~(1 << EORSTI);
+		if (user_reset)
+			user_reset();
+	}
 	flags = UEINT;
 	for (i = 0; i != NUM_EPS; i++)
 		if (1 || flags & (1 << i))
@@ -194,6 +200,13 @@ static void ep_init(void)
 }
 
 
+void usb_reset(void)
+{
+	UDCON |= 1 << DETACH;	/* detach the pull-up */
+	_delay_ms(1);
+}
+
+
 void usb_init(void)
 {
 	USBCON |= 1 << FRZCLK;		/* freeze the clock */