ESPNow gateway: outstanding issues

[peterhinch]

@glenn20 I have pushed an update that supports ESP8266. This aspect works in my testing. I'd appreciate feedback on the following.
[EDIT]

Port scanning (nodes)

This is now working on my PC but the code needs refining.

Error reporting (gateway)

Now implemented by publishing to a user-definable topic.

ESPNow message send integrity (gateway and nodes)

Currently the gateway implements the ACK mechanism to enable node publications to meet the qos==1 guarantee. This is predicated on the assumption that ESPNow.send() might return success when it has in fact failed. It is possible that, when sync==True, this condition cannot occur and that the node will always "know" if a send had succeeded. It would be good to lose ACK. What is your view?

Encryption

I haven't studied this in detail but it's on my todo list. I don't foresee any particular problem here.