init/Makefile: use wireless interface MAC by default

[Trek Hopton]

Since the Pi zero (non wifi) is no longer in production, we can assume devices running rv will have a wireless interface, therefore they will have a MAC address.

In the Makefile for rv, intead of using the default “00:E0:4C:00:00:01“ when no MAC is provided, we should get the devices MAC from the wireless interface. This way we can ensure that they are unique.

In some cases I have seen an error:

{"level":"warn","time":"2023-10-03T17:39:04.128+1030","caller":"netsender/netsender.go:349","message":"error in response","er":"device not found"}

This happens because the configured MAC is not set properly.