The major changes here are related to aiming to support building
FreeNAS and TrueNAS.  So start removing some of the "FreeNAS"
strings.

Also change the name of the base-os ackage to "base-os", and
the UI to "${NANO_LABEL}UI".

For the freenasOS library, start recording the name of the version
(aka avatar), and use that instead of hard-coded "FreeNAS".