WIP: RP2040 firmware

[dexterlb]

This PR bootstraps a the firmware for the on-board RP2040 chip (located on the Radxa X2L board) with the Raspberry Pi Pico SDK.

In this state, only the following features work:

• [X] USB and UART IO
• [ ] Firmware upgrade trigger
  • [X] Trigger via USB-CDC/UART
  • [ ] Implement the Vendor Reset interface (the one picotool uses)
• [ ] ST7789 display
  • [X] Basic interfacing to the display (writing pixels to the display)
  • [ ] Text font (writing text to the display
• [ ] Network switch interface
  • [X] PoC code (currently broken)
  • [ ] Fast implementation (PIO-based)
• [ ] Power board i2c interface
  • [ ] Fan controller
    • [ ] Expose as HID
  • [ ] GPIO extender
  • [ ] Temperature sensor
    • [ ] Expose as HID