UpdateFirmware.py should use flashDfu.py for the Photon

[elcojacobs]

The current script to update the firmware sends the new firmware over Serial. This does not work for the Photon, where also the system image should be updated.

The photon can be put into DFU mode automatically, so by invoking flashDfu.py --autodfu, the update script can perform the update automatically without user intervention.