Update render_doIdentityImageSyslinux.sh and
    render_doIdentityImageGrub.sh to separate the way we reduce image
    colors using pnmremap.
    
        - Previous to this commit, only Floyd-Steinberg dithering was
          applied when producing 14 and 16 colors images from full colored
          png images. This is good for most of situations. But in some
          cases, it is convenient to use just the same input color
          information, without Floyd-Steinberg dithering. 
    
        - In this commmit, both renderGrub and renderSyslinux
          functionalities were updated to receive options from
          pre-rendering configuration scripts in the form:
    
            ACTIONS[0]='renderImage'
            ACTIONS[1]='renderGrub:'
            ACTIONS[2]='renderGrub: -floyd'
    
          Making it possible to render images using Floyd-Steinberg
          dithering or not. 
          
          Option can be whatever option pnmremap supports, execpt -verbose
          and -map which are used already.
    
        - The final file names changed to:
    
            splash-14c.xpm.gz: 14 color image without Floyd-Steinberg
            dithering.
    
            splash-14c-floyd.xpm.gz: 14 color image with Floyd-Steinberg
            dithering.
    
            syslinux-splash-16c.png: 16 colors image without
            Floyd-Steinberg dithering.
    
            syslinux-splash-16c-floyd.png: 16 colors image with
            Floyd-Steinberg dithering.
    
    Update render_doIdentityImages.sh to use renderGrub and renderSyslinux
    new definitions. Both renderGrub and renderSyslinux definitions use
    one colon to delimit functionality names from options. If no option is
    passed, the last colon can be left out. 
    
        I'm woundering if using a space as character separator, between
        functionality names and options, would be a better idea.  I used
        the colon for renderFormats functionality initially, so used it
        here for consistency.
    
    Update Grub's and Syslinux's pre-rendering configuration scripts to
    use the new way of defining renderGrub and renderSyslinux
    functionalities (see description above).