submodule: bump openthread from 0abc8d7 to a54f4c4 (#533)

Bumps [openthread](https://github.com/openthread/openthread) from `0abc8d7` to `a54f4c4`.
- [Commits](openthread/openthread@0abc8d7...a54f4c4)

---
updated-dependencies:
- dependency-name: openthread
  dependency-type: direct:production
...

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