OTA: Major revamp

- Ditch ota command, no one uses it anyway lol
- Cleanup code
- Add support for device specific OTA processing (ota_preprocess.mokee for file processing, ota_postprocess.mokee
  for updater-script processing, place files in device tree)

Change-Id: Ie1f458b08bfaac63ab3cbde108d6b5df5389ea9f