buildinstall: Avoid pointlessly tweaking the boot images
    
    Only modify boot images if there actually is some change.
    
    The tweak function updates config files with volume id and kickstart
    file. Even if we don't have a kickstart and there is no change in the
    config files, the image will be regenerated. This leads to a change in
    checksum for no good reason.
    
    This patch keeps track of modified config files. If there are none, it
    avoids touching anything else.
    
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>