Enable advertising data or incoming connections

[EternityForest]

Recently on Reddit someone mentioned you can use the builtin speaker to help find a server in a server room. Finding something via BT could be useful or at least a fun demo in many battery powered boards, but there's also other interesting uses, like sending the current CPU temperature.

Being able to accept incoming Nordic Serial connection would also be interesting, for things like talking to the Pi from a web serial front-end.

Also interesting is the fact that advertising data would let us send messages to be received by another Pi, meaning we could in theory do low-bitrate mesh experiments.