Integrate firmware configuration with the Pico SDK's board definitions

[carlwhamilton]

This issue tracks the work needed to integrate the Debug Probe firmware's configuration and build process with the Pico SDK's board definition framework. As of SDK version 2.3.0, there is a board definition for the debug_probe hardware, which will be used when targeting that board (the default).

Real work on this effort will be tracked in this issue's sub-issues.

[carlwhamilton]

Nuts. I thought I could create this tracking issue, assign it to myself, and then add a couple of sub-issues to match pending PRs. Sadly, it doesn't look like I have the permissions to do that. ☹️

I have the following draft PRs that I would like send for review, the second not until the first has been pulled.

1. Move board selection logic into a dedicated header #208
2. Use the Pico SDK's board definitions in Debug Probe firmware #209

If using the issue tracking system in this way isn't the right approach, please let me know how you'd like me to contribute.