submodule: bump openthread from `5290a2a` to `a8b4fbb` (#282)
submodule: bump openthread from `5290a2a` to `a8b4fbb` (#282)

Bumps [openthread](https://github.com/openthread/openthread) from `5290a2a` to `a8b4fbb`.
- [Commits](https://github.com/openthread/openthread/compare/5290a2aed014f7ddcf19df7948e4f336e93c2681...a8b4fbb1893823e1c247ee1cc7dac8f61519cba4)

---
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-efr32,openthread/ot-efr32,openthread/ot-efr32