refactor: rearrange Kconfig inclusion order

[xudongzheng]

This makes it possible to override ZMK Kconfig options from shields.

This is useful for options such as ZMK_SPLIT_BLE_CENTRAL_PERIPHERALS, BT_MAX_PAIRED, BT_MAX_CONN, among others. Defining these in the shield Kconfig.defconfig works better than in the .conf since the they can be overridden from the user ZMK config repository.