Some misc notes on arduino-cli.

This is mostly UNTESTED. YMMV.

https://github.com/arduino/arduino-cli
    use arm32 + multiarch for version 0.3.6-alpha on arm64 hardware
apt install gcc-avr
arduino-cli core update-index
arduino-cli config init
nano -w ~/.arduino15/arduino-cli.yaml
board_manager:
  additional_urls:
    - https://raw.githubusercontent.com/adafruit/arduino-board-index/gh-pages/package_adafruit_index.json
    - https://raw.githubusercontent.com/sparkfun/Arduino_Boards/master/IDE_Board_Manager/package_sparkfun_index.json
    - https://dl.espressif.com/dl/package_esp32_index.json

arduino-cli core update-index
arduino-cli core install arduino:avr
arduino-cli core install arduino:sam
arduino-cli core install arduino:samd
arduino-cli core install adafruit:avr
arduino-cli core install adafruit:samd
arduino-cli core install SparkFun:avr
arduino-cli core install SparkFun:samd
arduino-cli core install esp32:esp32
arduino-cli core install esp8266:esp8266