Nonvolatile configuration

[brianbezanson]

Allow configuration parameters to persist through resets and add a "factory reset" command to restore the configuration to a known state. I see a few options for behavior:

1. Parameters are persistent by default.
2. Parameters are volatile by default, with a "use current values as defaults" command that makes the current parameter set persistent.
3. Parameters are volatile by default, with a per-parameter "set default value" command or command flag.

I think my favorite is 2.

[brianbezanson]

Configurable parameters:

• [x] Callsign
• [x] TX gate bias
• [x] TX VDD
• [x] PA current limit
• [x] PA voltage ramp delay
• [x] TX frequency
• [ ] RX frequency (later?)
• [x] RF bit rate
• [x] RF deviation (or roll that in with bit rate as a set of predefined combinations?)
• [x] TCXO pull voltage
• [ ] Modulation flags (CRC, gaussian filtering)

[brianbezanson]

In the configuration update command, include a config struct version number that increments whenever the structure changes. Reject commands containing incompatible config structs.

[iracigt]

Complete on settings branch! I left the frequency setting as one setting for both RX and TX. Modulation flags doesn't allow switching the Gaussian filter off. That can be made a separate issue if needed.