Migrate build target markers to keyboard.json

[zvecr]

Description

(Empty rules.mk) | (rules.mk with just comments) -> keyboard.json

Script used

for kb in $(qmk list-keyboards); do
    if [ -f keyboards/${kb}/info.json ]; then
        # ignrore blank lines and those starting with comments
        lines=$(cat keyboards/${kb}/rules.mk | grep . | grep -Ev "^#.*" | wc -l)
        if [[ $lines == 0 ]]; then
            git mv keyboards/${kb}/info.json keyboards/${kb}/keyboard.json
            git rm keyboards/${kb}/rules.mk
        fi
    fi
done

Only ~1300 keyboards still to migrate... 😢

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).