Support for other RP2040 boards

[Wenn0101]

Currently this core only has targets for the RP2040 connect and Raspberry Pi pico.

The getting started guide for the RP2040 lists 11 other RP2040 boards. Are there plans to add support for these other boards?

I am willing to do the work to add the 3 SparkFun targets.
I created a pull request to the rp2040 branch of mbed that adds these board definitions.