HardenedBSD Install Over SerialMon 13 August 2018
If you have APU or similar router, it's a big chance you'll need serial console install procedure. You'll have to mount install image before booting and change /boot/loader.conf so it includes the lines to use serial console:
boot_multicons="YES" boot_serial="YES" comconsole_speed="115200" console="comconsole,vidconsole"
When the machine boots, it will ask you for prefered console type. Default (vt100) is just fine. The rest of the installation is just like on the normal machine, but you'll have to modify /boot/loader.conf on the newely installed. Reboot, and boot off of USB key once again with the same procedure, but go into shell, instead of install.
mkdir /tmp/install zpool import -R /tmp/install -f zroot zfs mount zroot/ROOT/default
Now write the same lines for serial console to /tmp/install/boot/loader.conf and you should be set.