Best posts made by perIpI

I edited according to Yveaux's work around and its Alive! Connected to the local WIFI OK.

Im not home at the moment so integration with Openhab will have to wait. Thank you all for your help!

@electrik

Tested and worked as well. I think this is preferable over the first solution (forward function declaration) as it gives a reference where the functions are defined. Thank you!

Replying myself after more reading and testing.
*MQTT vs ETH probably is equivalent in my system, if the choice if given I think MQTT is preferred.
*NRF24 or RFM69, maybe both as I already have a bunch of NRF24 radios home and can start build directly.
*PC based MQTT could be an idea, but probably its as easy to work through an Arduino-example using the serial monitor in an Arduino-environment.

My way forward will be:
Remove the RPi-GPIO-gateway (I built a basic node and tested it, but could not communicate with the GW) and replace it with ESP8266-based WIFI/MQTT/Radio-gateway (one with NRF24 and eventually one with RFM69).

I believe that will be an easier way into mySensors-world also limiting the amount of required Linux-related knowledge. It will be easier to debug the system from Arduino IDE-serial console. Ie start and confirm OH3 to MySensors-MQTT-gateway, then proceeding with sensor to gateway communication etc.