Check to see if ports-mgmt/poudriere-devel port is installed before building.
Set NANO_XZ to pxz by default.  do_build.sh checks to see if
pxz is installed and errors out if it is not.
Simplify the script, since it is only used to build a target named "os-base".
Split nanobsd.sh into two scripts:
   - nanobsd_funcs.sh  -> This script contains functions and variables.
                          This script should be sourced in other scripts
   - nanobsd.sh        -> This script sources nanobsd_funcs.sh and
                          runs commands.

We will be able to write multiple scripts which call
different subsets of nanobsd_funcs.sh.  For example, a script
which just builds the world/kernel.
Add.
Add a script which builds the jail which we will use to build ports.
Move some variables from os-base to nano_env.
That way we can source nano_env to set these variables in other scripts.
If we source os-base, we end up executing commands in os-base.
Move FlashGeneric back to os-base
Remove source of os-base.  We don't need it here.
Comment out CONF_BUILD, CONF_WORLD, and CONF_INSTALL.
They need to be set in nano_env.
Add some logic from nanobsd.sh script for enabling "make -j" for
parallel builds.
Remove prepare-ports-env target, combine it into single ports target.