Move SPLIT_KEYBOARD to data driven

[dunk2k]

Description

In light of @fauxpark's ongoing efforts to iteratively modernize the entire keyboards directory to data driven format. Rather than it be a one man effort, I'd chip in by moving SPLIT_KEYBOARD from rules.mk to info.json.

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).

[noroadsleft]

Thanks!