Add 'arduino' (portduino) as an optional build target

[geeksville]

This PR is just a draft for now, once portduino gets a bit more mature I can send it in if you want it.

Mostly adding this draft so that interested parties can discuss the idea here: https://community.platformio.org/t/portduino-a-new-platform-framework-to-allow-running-arduino-code-on-linux-feedback-requested/22968