Fix issues with data driven split keyboards

[zvecr]

Description

Using the following qmk new-keyboard -t atmega32u4 -kb asdf -u zvecr -n asdf -l split_3x6_3 and info.json updated to:

{
    "manufacturer": "asdf",
    "keyboard_name": "asdf",
    "maintainer": "zvecr",
    "bootloader": "atmel-dfu",
    "diode_direction": "COL2ROW",
    "matrix_pins": {
        "cols": ["C2", "C2", "C2", "C2", "C2", "C2", "C2"],
        "rows": ["D1", "D1", "D1", "D1"]
    },
    "split":{
        "enabled": true,
        "soft_serial_pin": "D2",
        "matrix_pins": {
            "right": {
                "cols": ["C2", "C2", "C2", "C2", "C2", "C2", "C2"],
                "rows": ["D1", "D1", "D1", "D1"]
            }
        }
    },
    "processor": "atmega32u4",
    "usb": {
        "device_version": "1.0.0",
        "pid": "0x0000",
        "vid": "0xFEED"
    },
    "layouts": {
        "LAYOUT_split_3x6_3": {
            "layout": [
                { "matrix": [0, 0], "x": 0, "y": 0.25 },
                { "matrix": [0, 1], "x": 1, "y": 0.25 },
                { "matrix": [0, 2], "x": 2, "y": 0.125 },
                { "matrix": [0, 3], "x": 3, "y": 0 },
                { "matrix": [0, 4], "x": 4, "y": 0.125 },
                { "matrix": [0, 5], "x": 5, "y": 0.25 },
                { "matrix": [4, 0], "x": 8, "y": 0.25 },
                { "matrix": [4, 1], "x": 9, "y": 0.125 },
                { "matrix": [4, 2], "x": 10, "y": 0 },
                { "matrix": [4, 3], "x": 11, "y": 0.125 },
                { "matrix": [4, 4], "x": 12, "y": 0.25 },
                { "matrix": [4, 5], "x": 13, "y": 0.25 },
                { "matrix": [1, 0], "x": 0, "y": 1.25 },
                { "matrix": [1, 1], "x": 1, "y": 1.25 },
                { "matrix": [1, 2], "x": 2, "y": 1.125 },
                { "matrix": [1, 3], "x": 3, "y": 1 },
                { "matrix": [1, 4], "x": 4, "y": 1.125 },
                { "matrix": [1, 5], "x": 5, "y": 1.25 },
                { "matrix": [5, 0], "x": 8, "y": 1.25 },
                { "matrix": [5, 1], "x": 9, "y": 1.125 },
                { "matrix": [5, 2], "x": 10, "y": 1 },
                { "matrix": [5, 3], "x": 11, "y": 1.125 },
                { "matrix": [5, 4], "x": 12, "y": 1.25 },
                { "matrix": [5, 5], "x": 13, "y": 1.25 },
                { "matrix": [2, 0], "x": 0, "y": 2.25 },
                { "matrix": [2, 1], "x": 1, "y": 2.25 },
                { "matrix": [2, 2], "x": 2, "y": 2.125 },
                { "matrix": [2, 3], "x": 3, "y": 2 },
                { "matrix": [2, 4], "x": 4, "y": 2.125 },
                { "matrix": [2, 5], "x": 5, "y": 2.25 },
                { "matrix": [6, 0], "x": 8, "y": 2.25 },
                { "matrix": [6, 1], "x": 9, "y": 2.125 },
                { "matrix": [6, 2], "x": 10, "y": 2 },
                { "matrix": [6, 3], "x": 11, "y": 2.125 },
                { "matrix": [6, 4], "x": 12, "y": 2.25 },
                { "matrix": [6, 5], "x": 13, "y": 2.25 },
                { "matrix": [3, 3], "x": 3.5, "y": 3.25 },
                { "matrix": [3, 4], "x": 4.5, "y": 3.5 },
                { "matrix": [3, 5], "x": 5.5, "y": 3.75 },
                { "matrix": [7, 3], "x": 7.5, "y": 3.75 },
                { "matrix": [7, 4], "x": 8.5, "y": 3.5 },
                { "matrix": [7, 5], "x": 9.5, "y": 3.25 }
            ]
        }
    }
}

This PR contains fixes for:

• rules.mk incorrectly generating SPLIT_KEYBOARD ?= on
• Matrix data out of bounds for layout LAYOUT_split_3x6_3 at index 6 (k4A): 4, 0
• config.h declaring matrix size incorrectly

This at least produces a working config that can be iterated on.

Types of Changes

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

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).

[sigprof]

Do you still remember why this code checks 'matrix_pins' in kb_info_json, and not 'matrix_size' in kb_info_json? With the matrix_pins check it is impossible to specify the matrix size in info.json if a custom matrix is used, and therefore matrix_pins is not defined.