Make "node-terkin-next" build on ESP32

[amotl]

Hi there,

we tried to roughly sketch out a Firmware based on Terkin-Next (WIP), see also [1]. This might resonate with @marcelgasser.

While working on #39, we found the code [2] would not compile on ESP32. The reason is that the code has been conceived in 2016 already. It would be nice if we could make it work.

With kind regards,
Andreas.

[1] https://hiveeyes.org/docs/arduino/firmware/node-terkin-next/README.html
[2] https://github.com/hiveeyes/arduino/tree/master/node-terkin-next