Support firmware compile for multiple boards...

Support firmware compile for multiple boards, _e.g., `mega2560`, `uno`_.
__NB__ Currently, the firmware is much larger than what can be supported
on the `uno`.  However, once ARM build support is added, we could target
the [`due`][1], along with the current default board
_([`mega2560`][2])_.

Remove `avr` and `serial_device` modules.  Add [`avr_helpers`][3] and
[`arduino_helpers`][4] as dependencies, instead.

[1]: http://arduino.cc/en/Main/arduinoBoardDue
[2]: http://arduino.cc/en/Main/arduinoBoardMega2560
[3]: https://github.com/wheeler-microfluidics/avr_helpers
[4]: https://github.com/wheeler-microfluidics/arduino_helpers