Board Selection for Configurator

[jbarket]

The web configurator is driven by Boards.json, and the activate board is set in .env. The problem is that there's no simple way to have that board set by changing the board configuration files in the main firmware. We could do something at build time to change the .env values, but that seems unnecessary.

Instead, I think we should set that information in the BoardConfig.h file. In the configurator, we should pull that value and use it to determine which board in Boards.json to use.

Then we can reimplement the pin restrictors for the Pico, add a full RP2040 profile for most custom boards that just use every pin, and allow the creation of custom boards for anything that doesn't fit those two.