Add bluetooth support for obtaining NMEA

[gdt]

Separately from NTRIP, it would be very useful to enable accessing the NMEA stream over bluetooth. This would ease integration with various devices, especially software that already has bluetooth support, which seem to be the standard approach for "external GPS".

(While it would also be nice to have a way for a computer with bluetooth to proxy NTRIP, this request is just about access to the NMEA output stream.)