According to a Usenet posting forward to me by Jordan, there's

at least one user out there who's system won't autoboot from the
serial console because of what sounds like 'phantom keystrokes'
making the timeout timer trip. I've tried to solve this by
adding an extra call to init_serial() right before the 'Boot:'
prompt is printed (done only if RB_SERIAL is set) to hopefully
make sure that the input buffer is clear. Unfortunately, the fellow
is in Germany and I haven't heard back from him yet. I haven't
been able to duplicate this problem on my hardware, so this is
a stab in the dark. At the very least, it shouldn't hurt anything.

sys/i386/boot/biosboot/boot.c

@@ -24,7 +24,7 @@
  * the rights to redistribute these changes.
  *
  *	from: Mach, [92/04/03  16:51:14  rvb]
- *	$Id: boot.c,v 1.32 1995/01/28 03:51:39 davidg Exp $
+ *	$Id: boot.c,v 1.33 1995/03/14 08:21:53 davidg Exp $
  */
 
 
@@ -109,6 +109,12 @@ Use ? for file list or press Enter for defaults\n\n",
 	maj = (drive&0x80 ? 0 : 2);		/* a good first bet */
 
 	printf("Boot: ");
+	/*
+	 * Be paranoid and make doubly sure that the input
+	 * buffer is empty.
+	 */
+	if (loadflags & RB_SERIAL)
+		init_serial();
 	getbootdev(&loadflags);
 	ret = openrd();
 	if (ret != 0) {