fedora-live-base: Handle livecd-creator vs. livemedia-creator builds
    
    Test for $LIVE_ROOT (set by livecd-creator) before running commands
    specific to a livecd-creator build.
    
    Do not modify the Lorax template during livemedia-creator builds;
    the change belongs in Lorax itself.
    
    Related: https://github.com/weldr/lorax/pull/1107
    
    Resolves: rhbz#1430546