Investigate possibility of communicating with the Pi through UART1

[DrLex0]

One of the motivations for using the buzzer as a communication channel was that it doesn't require installing a custom firmware. However, due to the annoying heaters tune played by recent Sailfish builds, a custom firmware build is needed anyway to remove the risk of this tune messing up beep sequences. The pull request to toggle the tune might some day be merged into Sailfish, but it could take forever before this ends up in firmware that the average Joe can easily install.

Hence if (for the time being) a special Sailfish build needs to be flashed anyway, the possibility should be investigated whether an extra X3G command could be added that can send short strings to the second serial output (UART1) of the MightyBoard. This would provide a more reliable communication (and silent) channel to the Raspberry Pi, and open up the possibility to control many other external devices from within a print.