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.