MQTT backend

[obones]

Since RFLink decided to close the source for their firmware, there has been an effort to continue the project from the latest public sources, and at the same time bring RFLink to ESP8266/ESP32
The most active repository on that front is here: https://github.com/cpainchaud/RFLink32

It brings in Ser2Net but also the ability to publish to a MQTT broker.

I believe it would be nice if that binding would be capable of subscribing to an MQTT topic as well as keeping the serial port handling.