#append these 3 lines into /boot/loader.conf
boot_serial="YES"
comconsole_speed="115200"
console="comconsole"

# vi /etc/ttys: comment out old, use new
ttyu0	"/usr/libexec/getty std.115200"	vt220	on secure
#ttyu0	"/usr/libexec/getty 3wire"	vt100	onifconsole secure