Bug 188549

I have a PC Engines APU platform that has no video console but only a
serial one. Although there is a brief documentation in the handbook on
how to get the installation image working on the serial console
(https://www.freebsd.org/doc/handbook/install-advanced.html) a lot of
things are missing or troublesome to get a properly working system
manageable over the serial console only:

- Installation images have to be modified by hand to get serial console
  support
- Serial console is not enabled in the installed system, making it unusable
  for me
- Booting the installation image again and entering the rescue shell gives
  me no easy way to mount the installed system if it is on ZFS. Importing/
  exporting the pool is not very intuitive.
- Enabling SSHD during installation is an option. Later you can login over
  network and fix the serial console. But this relies on configuring a
  non-root user during installation and giving him the wheel group,
  otherwise you can not become root on the installed system over SSH.

Fix: 

- Provide pre-built installation images with serial console support
- Provide option in installer to enable serial console on installed system
- Provide documentation on how to fix a system on ZFS with the rescue shell
- Provide option/documentation on how to configure user(s) to be able to
  fix installed system via SSH
How-To-Repeat: - Install fresh system on hardware with serial console support, put root
  on ZFS and enable SSHD
- Boot into the installed system, check serial console login
- Reboot to rescue shell, try configuring serial console
- Try logging in via SSHD, becoming root and configuring serial console