Board identification improvements

[cmaglie]

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• [x] The PR has no duplicates (please search among the Pull Requests before creating one)
• [x] The PR follows our contributing guidelines
• [x] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)
• [ ] UPGRADING.md has been updated with a migration guide (for breaking changes)

What kind of change does this PR introduce? This PR adds the possibility to the board list command to detect the board FQBN with full configuration (the configuration it's the last portion of the FQBN i.e.: arduino:avr:mega:CONF=VALUE)

What is the current behavior? The board list command can not detect the board configuration.

What is the new behavior? The board list command may return the full FQBN including the board configuration if the platforms correctly support it.

Does this PR introduce a breaking change, and is titled accordingly? The fqbn returned from the board list command now may contain also the config part arduino:avr:mega:CONF=VALUE,.... Strictly speaking, it's not a breaking change, since the FQBN is supposed to contain this (optional) configuration part. BTW this is very uncommon so the clients parsing the FQBN may not be ready to handle this optional piece.

Other information: Some other improvements on subroutines handling boards metadata have been made, with many pieces refactored to improve source code readability.