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

(cherry picked from commit c6a2bfeec066725730c03ff9ad88cd84173400d7)
Disable (unused) serial port interrupts during console initialization.

Without this change BIOS on EchoStream box does not always release the
SOL input from BIOS redirection to the loader. It is not very logical,
but seems working.
Merge remote-tracking branch 'origin/feature/unified_freebsd' into feature/hyperv