Allow to add more boards to existing platformio.ini

[gandy92]

In a fresh directory, platformio init --board A --boardB will create platformio.ini with environments for board A and board B. If the file platformio.ini already exists, we get an error:

Error: Project is already initialized. Process it with `platformio run` command

During development of a new project, it may be necessary, to add new environments supporting different boards. In that case, the user has to fill in all required fields manually, wihtout the benefit of the mechanims used with init command.

IMHO, it would be more user-friendly to provide a new command or extend the init command so that the user can add further boards to the platformio.ini.

[ajford]

I agree with this. I ended up copy/pasting the existing entry and changing manually, but it would be nice if it was possible to do this via the interface.