Help is available by running farbot -h:

Usage: ./farbot [-h] [-o] [-f config file] [-r action]
    -h             Print usage (this message)
    -o             Do one action only.  Do not continue after <action>
    -f <config>    Use configuration file <config>
    -r <action>    Execute <action>

Supported actions:
    release        Build all defined releases, build all packages, and build the
                   network installation root
    package        Build all defined packages, and build the network 
                   installation root (requires a release build)
    install        Build the network installation root (requires package and 
                   release builds)

./farbot -f farbot.conf -r release

./farbot -f farbot.conf -r install