Support multi-licensed packages

[2bndy5]

I noticed the license field of the library.json only takes a string with 1 SPDX identifier. I have a lib that distributes with 3 different licenses (all have a SPDX identifier). I was wondering how to go about specifying this in the JSON (if possible). I would have guessed that the license field could optionally take a list of strings (all SPDX identifiers) in this scenario.

In the Meantime I have resorted to only specifying 1 of the 3 licenses in the JSON (its not published to PIO registry yet).

[ivankravets]

See updated docs for https://docs.platformio.org/en/latest/librarymanager/config.html#license

You need to use SPDX Expression.