Excerpts from netbsd: 23-Feb-98 Question about biosboot.sym.. Brian
Buhrow@cats.ucsc.e (659)

> I tried defining -DSUPPORT_SERIAL and -DDIRECT_SERIAL,
> along with the rest of the stuff on that line, including setting the baud
> rate to 9600.  (see src/sys/arch/i386/stand/biosboot/Makefile for details),

That looks well so far... If you always want a serial console,
you probably don't want COMCONS_KEYPRESS - this switches
to the serial console if you press a key in the right moment.
Did you change the initio() argument in main() to CONSDEV_COM0?

>	Is there any documentation on how to enable this feature?

That's of course all well documented. (grin)
Look at boot_console(8).

best regards
Matthias