Enable 'keyboard.json' as a build target

[zvecr]

Description

Long term QMK wants to support a single data file for keyboard level config.

The current system requires a rules.mk even if the content is completely blank. The plan is to have info.json as potential fragments and add support for a new keyboard.json (supporting all info.json content) to mark a build target.

Achieving this goal will take multiple passes in order to not break everything in the short term and to keep the change sets reviewable. This first pass enables keyboard.json to exist so we can start the overall migration process.

Future:

• Refactor keyboards to use keyboard.json
• Remove use of DEFAULT_FOLDER
• Remove rules.mk as build marker
• Tidy up info.json code/docs references now 2 potential filenames exist

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[github-actions]

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