loader: comconsole: don't unconditionally wipe out hw.uart.console

It may be the case that we need to set hw.uart.console manually in some
scenarios that comconsole can't necessarily support. Avoid clobbering
hw.uart.console unless we've actually selected comconsole so that one
could at least get kernel console output..

Discussed with: imp
Sponsored by: Zenith Electronics LLC
Sponsored by: Klara, Inc.

(cherry picked from commit ec671f4980437008b0119c6367816083e15fa503)
(cherry picked from commit 362677cae8e9ac4c6f7ff4d74a4378ea11d47797)

Approved by: so
Security: FreeBSD-EN-23:06.loader