Add physical safe shutdown switch to Stratux FLARM

I have seen the idea to support a physical switch to trigger the shutdown command on your Stratux FLARM, powered by a Raspberry PI 3 or 4, rather than just cutting the power which will most likely result in issues with the micro SD card.

To achieve this, I have modifed the offical Stratux Firmware to support out of the box a a physical switch

physical switch must be connected at GPIO pin 39 GPIO pin 39 (Ground) and 40 (GPIO 21 – PCM_DOUT)