Some years ago I developed a couple of boards for the MySensors project, sensebender micro based on atmega328 and sensebender gateway based on atsamd21.
How can we get these boards supported in PlatformIO, specifically the samd21? Our board definitions for arduino are placed here GitHub - mysensors/ArduinoBoards: Mysensors hardware Package Lists for the Arduino Board Manager, and the source code for the samd21 is in a separate repo GitHub - mysensors/ArduinoHwSAMD: Arduino hardware definitions for SAMD platform.
It’s to a large extent based on the arduino zero, but the pin mappings are different, so it needs it’s own config files for setting up the peripheral matrix. Also some board specific pin defines are made in the .h file.