prepare-root: Refactor composefs config handling

prepare-root: Refactor composefs config handling

- Convert the current enum into a struct, using `OtTristate` and
  two member variables (expected signature and digest)
- Factor out a helper function to parse this config
- Clean up the logging by consistently using `composefs:` as a prefix
- Add more assertions to more strictly verify our runtime state
  since this is security relevant