Update centos-art.sh script to introduce BASE, POST, and LAST
    rendering-actions concept as value to ACTIONS array variable, inside
    pre-rendering configuration scripts. These definitions let you apply
    actions in three different times.
    
        - The `BASE' action specifies what kind of rendering does the
          centos-art.sh script will perform with the files related to the
          pre-rendering configuration script. The `BASE' action is
          required. Possible values to `BASE' action are either
          `renderImage' or `renderText' only. 
    
        - The `POST' action specifies which action to apply for each file
          rendered (at the rendering time). This action is optional. You
          can set many different `POST' actions to apply many different
          actions over the same already rendered file. Possible values to
          `POST' action are `renderFormats', `renderSyslinux',
          `renderGrub', etc.
    
        - The `LAST' action specifies which actions to apply once the last
          file in the list of files to process has been rendered. The
          `LAST' actions are optional. Possible values for `LAST' actions
          may be `groupByFormat', `renderGdmTgz', `renderKSplash', etc.
    
    Unify some awk scripts into their own files. This let us reuse them
    and reduce maintainance.