* machine: add support for BTT SKR Pico
Adds support for the BigTreeTech SKR Pico 3D-printer mainboard.
This board uses the RP2040.
* Fix build tag
* Add I2C defaults
* Run UART test instead of blinky1
* Use NoPin for I2C and SPI on BTT SKR Pico
* Cleanup comments
* Don't use ADC pin names