Radio

This branch adds Radio support to the badge:
* Starts in discovery mode
* Keeps track of strongest signal
* Connects to main channel
* Receives packets
* Combines packets into messages
* Checks message via ECC against public key
* Deserialises weather forecast data
* Stores data in memory for other tasks to use

There is no support for badge-specific messages or any way of sending messages back yet. The radio task is currently using busy waiting.

Signature verification takes about 150ms per message at the moment.