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/boot.c

@@ -14,6 +14,7 @@
 #include <stdint.h>
 
 #include <avr/io.h>
+#include <avr/interrupt.h>
 #include <avr/pgmspace.h>
 
 #define F_CPU   8000000UL
@@ -50,10 +51,15 @@ int main(void)
 	usb_init();
 	dfu_init();
 
+	/* move interrupt vectors to the boot loader */
+	MCUCR = 1 << IVCE;
+	MCUCR = 1 << IVSEL;
+
+	sei();
+
 	led(1);
 
 	while (loop != MS_TO_LOOPS(2000)) {
-		usb_poll();
 		if (dfu.state == dfuIDLE && pgm_read_byte(zero) != 0xff)
 			loop++;
 		else
@@ -62,6 +68,8 @@ int main(void)
 
 	led(0);
 
+	cli();
+
 	usb_reset();
 	run_payload();