Always update the hw.uart.console hint anytime a change is made to the
comconsole setup. Previously the hint would be set when if you set a
custom port, but it would not be updated if you later set a custom speed.

Also, leave the hw.uart.console hint mutable so it can be overridden or
unset by the user if needed.

Reviewed by: kib (earlier version)
MFC after: 1 week