Added support for hidden boards in 'board listall' command

[cmaglie]

For example in the arduino:samd platform:

$ arduino-cli board listall MIPS
Board Name FQBN 

$ arduino-cli board listall MIPS -a
Board Name                       FQBN                                               
Arduino Tian (MIPS Console port) arduino:samd:tian_cons                     (hidden)

$ arduino-cli board listall Tian -a --format json
{
  "boards": [
    {
      "name": "Arduino Tian (MIPS Console port)",
      "FQBN": "arduino:samd:tian_cons",
      "is_hidden": true
    },
    {
      "name": "Arduino Tian",
      "FQBN": "arduino:samd:tian"
    }
  ]
}