grubby-bls: fix corner case when a kernel param value contains a '='
    
    If a kernel command line parameter has a '=' character, grubby fails
    to update the options field in the BLS snippet, for example:
    
    $ grubby --update-kernel=foo --args="bar=https://test?token="
    sed: -e expression #1, char 30: unknown option to `s'
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>