Integrate with tflite-micro's create_tflm_tree.py script

As per https://github.com/tensorflow/tflite-micro/issues/407, use the create_tflm_tree.py or similar to set up a build tree.

[create_tflm_tree.py](https://github.com/tensorflow/tflite-micro/blob/main/tensorflow/lite/micro/tools/project_generation/create_tflm_tree.py) cause the
TfLM makefile to print out the reference source and header files (via targets list_library_sources and list_library_headers). It also optionally fetches downloads examples and copies example source code to correct paths.

My initial thoughts are that it would be useful to call create_tflm_tree.py directly, since that script will be maintained by the TfLM team.

There is some documentation in the tflite-micro repository at [new_platform_support.md](https://github.com/tensorflow/tflite-micro/blob/main/tensorflow/lite/micro/docs/new_platform_support.md). It encourages us to add our CI to tflite-micro's [community supported builds list](https://github.com/tensorflow/tflite-micro#community-supported-builds) once we are using this new mechanism.