Persist logs on terminate if appropriate.

[taun-fellowes]

If the logs are being persisted periodically, then it would also be useful if they are persisted as part of a terminate event to avoid losing logs to a Nerves.Runtime.reboot/0.

[fhunleth]

Hi @taun-fellowes! Thanks for the PR. It makes sense to me.

Would it be possible to add a small unit test?

Also, don't worry about CI failures on Elixir 1.11 or 1.12. I need to make a pass at updating this project for Elixir 1.17. I assume it will be a trivial update since I know Elixir 1.17 works fine, but a side effect will removing support for those older versions.

[fhunleth]

Thanks! Also, a unit test is coming in the next commit.