Use an SD card rather than FRAM

[jeff-winn]

As a user, I would like the ability to set my own pin for the Bluetooth connection bonding without having to compile the software myself. Because the current mechanism requires the use of the FRAM chip, and the difficulty to set it up is high having an externalized SD card I can plug into my computer to set the value in would be extremely helpful.

[jeff-winn]

Potential hardware:
https://www.adafruit.com/product/4682

This appears to also require the use of the SD library since it's SPI based, nor does the Feather M4 Express seem to support SDIO.

[jeff-winn]

The configuration settings class in the master device will likely need to absorb this change in technology.

Still not sure whether JSON or an INI style file should be used.