MLBEDSW-10293: TFLite buffer offset support

- Regenerated TFLite schema with mutable API
- TFLite reader: implement mechanism to load buffers at the end
  of the file as described in the schema
- Update vela.py to read via mmap
- TFlite writer: implement mechanism to write buffers at the end of the
  file as described in the schema

Change-Id: I169a5f0e512f1b038393145495ec7040be783969
Signed-off-by: Mauricio Briceno <mauricio.briceno@arm.com>