Enhancement: Implement a config file on SD

[conr2286]

Configurable options might someday include:

• Station callsign
• Grid square or "gps"
• Operating frequency (within limits of the LP filter)
• SOTA summit
• Optional CQ info (e.g. CQ POTA or whatever)
• Frequency correction for TCXO
• Extra info for log file???

[conr2286]

Initial Implementation on main. The SD config.json file has potential applications for debugging as well as station operation.