RN2903 AU firmware

Unfortunately Microchip have still not released their AU firmware yet. We are all waiting patiently (or impatiently) for this to happen and I understand it will still be a while.

Multitech mDot/xDot have had AU firmware available for almost a year now.

ADR is only implemented in EU at the moment, but looks like it’s coming soon (hopefully). See https://github.com/TheThingsNetwork/ttn/issues/619.