atusb/fw/: changed USB stack to use interrupts instead of polling

Note: this change surprisingly _increases_ the DFU wait in the boot
loader. Not yet sure why.

- boot.c (main): move the interrupt vectors to the boot loader
  section
- atusb.c (main): move the interrupt vectors to the application
  section
- boot.c (main): enable global interrupts while looping (disable
  them before jumping to the application)
- board_app.c (__timer_read, timer_read): removed wrapped since
  we're now always called with interrupts disabled
- usb/atu2.c (ep_init): enable endpoint interrupts
- usb/atu2.c (usb_init): enable device interrupts
- usb/atu2.c (usb_poll, USB_GEN_vect, USB_COM_vect): moved poll
  loop code into separate handlers for device and endpoint
  interrupts
- boot.c (main), atusb.c (main): removed call to usb_poll

atusb/fw/atusb.c

@@ -37,8 +37,11 @@ int main(void)
 	ep0_init();
 	timer_init();
 
+	/* move interrupt vectors to 0 */
+	MCUCR = 1 << IVCE;
+	MCUCR = 0;
+
 	sei();
 
-	while (1)
-		usb_poll();
+	while (1);
 }