UFS in a USB flash drive (ufs:/dev/da0s1a)

 Hi,

I have a scenario (see below) after which the FreeBSD kernel boots-up in my
embedded system, it is looking for the root filesystem. Now, I want to
build one from scratch based on the examples below using a UFS filesystem
in a USB flash drive -> ufs:/dev/da0s1a. Are there any available
step-by-step reference(s) that can help and guide me in the building
process?

...
Trying to mount root from cd9660:/dev/map/rootfs.uzip []...
mountroot: waiting for device /dev/map/rootfs.uzip...
Mounting from cd9660:/dev/map/rootfs.uzip failed with error 19.

Loader variables:

Manual root filesystem specification:
  <fstype>:<device> [options]
      Mount <device> using filesystem <fstype>
      and with the specified (optional) option list.

    eg. ufs:/dev/da0s1a
        zfs:tank
        cd9660:/dev/cd0 ro
          (which is equivalent to: mount -t cd9660 -o ro /dev/cd0 /)

  ?               List valid disk boot devices
  .               Yield 1 second (for background tasks)
  <empty line>    Abort manual input

mountroot>
...

Thanks and best regards,
Archimedes