more sophisticated ways for updating the atusb firmware

[Werner Almesberger]

Stefan Schmidt wrote:
> Have you considered using the standard DFU method to signal a firmware
> upgrade during normal operation?

Finally implemented it. Works like a charm :-)

I also increased the boot loader delay from 2.0 s to 2.5 s to avoid
racing with dfu-util.

So there are now three approaches for updating the application:

- the old way (make update):

   atrf-reset -a
   # wait until the device enumerates or about 1 s
   dfu-util ...

- the proper way (make dfu):

  dfu-util ...

- the brick way:

  # unplug and re-connect the atusb dongle
  # wait, as above
  dfu-util ...

Boot loader and application still share the same PID. We can change
the application's PID with a firmware update in the field, should
the need arise.

- Werner