add a power on flash

[pljakobs]

When the controller is in a closed case, it's hard to see if a reboot was successful.
It would be good to have a short light signal, to show that the firmware has initialized and maybe connected to the network.
This could be implemented as a configurable mode so the old behavior is kept the default.
Taking the idea a bit further, different light signals could be defined to help debug the controllers current state like, probably the most typical error, unable to connect to wlan, but also things like states of OTA etc.