grubby-bls: make a copy of the cmdline if is modified for an entry
    
    The BLS entries use a global kernel command line parameters that's set as
    kernelopts variable in the grubenv file. But users may want to have custom
    parameters for some entries, so make a copy of the current kernelopts and
    set the "options" field value to the modified parameters.
    
    Resolves: rhbz#1629054
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>