submodule: bump openthread from `515c3d3` to `4bd7ab4` (#259)
submodule: bump openthread from `515c3d3` to `4bd7ab4` (#259)

Bumps [openthread](https://github.com/openthread/openthread) from `515c3d3` to `4bd7ab4`.
- [Commits](https://github.com/openthread/openthread/compare/515c3d3d80fe374011cb0577e4fbc5ad6393be45...4bd7ab4b798728d8267fbc92c0db970e5799d210)

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

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>

Co-authored-by: dependabot[bot] <1c358da00a777d4e9898c1280ab801e2df165188@users.noreply.github.com>
openthread/ot-samr21,openthread/ot-samr21