Please check if the PR fulfills these requirements

• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• The PR follows
  our contributing guidelines
• 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?
  Added two fields board_platform and build_platform in the compile JSON output.

board_platform is the platform where the board is defined.
build_platform is the platform used to build the sketch (it may be different if the board_platform reference a core from another platform)

$ arduino-cli compile -b arduino:avr:uno --format json
{
  "compiler_out": "Sketch uses 2406 bytes (7%) of program storage space. Maximum is 32256 bytes.\nGlobal variables use 1212 bytes (59%) of dynamic memory, leaving 836 bytes for local variables. Maximum is 2048 bytes.\n",
  "compiler_err": "",
  "builder_result": {
    "build_path": "/tmp/arduino-sketch-3A27009D252C3EBEA95BE31BD605BF0D",
    "executable_sections_size": [
      {
        "name": "text",
        "size": 2406,
        "max_size": 32256
      },
      {
        "name": "data",
        "size": 1212,
        "max_size": 2048
      }
    ],
    "board_platform": {
      "id": "arduino:avr",
      "version": "1.8.4"
    },
    "build_platform": {
      "id": "arduino:avr",
      "version": "1.8.4"
    }
  },
  "success": true
}

• Does this PR introduce a breaking change, and is
  titled accordingly?
  No

• Other information:
  Slightly refactored the way commands.Compile returns the result.