This setup is designed for an image that contains an unaltered Kafka distribution. It uses a ConfigMap + init container instead of a custom image entrypoint script.
A caveat is that the ConfigMap isn't part of the StatefulSets rollout. To remedy this, once your're happy with your init script (outside service names etc.) build your own kafka image that uses your script as entry point and remove the init container.
Note that brokers depend on Zookeeper.