Deprecated or generalize the 'partial support' concept in apio.

[zapta]

This was transfered from #461

This issue is for the deprecation or the generalization of the 'partial support' feature in apio. That is, fpgas, programmer, and boards that are supported only on some of the platrorms.

Deprecation option:
With this model, all boards, fpgas, and programmers are supported on all apio supported platform. The change is simple

• Delete from boards.json the (two) boards with platform: attribute.
• (optional) Add to apio examples example(s) for the deleted boards with custom boards.json in their project dir.
• Remove the code that is related to the boards.json platform: property.

Generalization option:

• Define a way to specify in .json files a subset of the supported platform. Ideally it will allow to substruct from the set of all platforms and to add to the empty set of platforms. e.g. mutually exclusive fields supported_platforms: ["windows_amd64", "darwin"] and non_supported_platforms: ["windows_amd64", "darwin"]
• Allow to use these specification in boards.json, fpgas.json, and programmers.json.
• In any project related apio command, warn in the begining if the current project is not fully supported on the current platform. The goal is to avoid surprises at latter stages of a project.