grubby-bls: fix logic to check if the kernelopts var is defined in a BLS
    
    The logic to check if a kernelopts variable is defined in a BLS entry was
    wrong and caused grubby to expand the variables even when it was defined.
    
    Resolves: rhbz#1726514
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>