systemd ships `units/serial-getty@.service.m4` with the `--keep-baud`
option.
We override that unit, and didn't add the `--keep-baud` option. (We have
it in our other getty options there).
Having `--keep-baud` in `serial-getty@` makes a lot of sense - the
console keeps working if it's initialized with a less standard baud
rate, such as the [Helios64](https://wiki.kobol.io/helios64/intro/).
(cherry picked from commit ba42d639f16dc774f4fa661243b640b034d7be0a)
f4b92c2975