grubby-bls: expand only the kernelopts variable
    
    The BLS snippets have the options field set to a kernelopts variable by
    default, which is set in GRUB's environemnt block to the kernel cmdline.
    
    When the kernel command line parameters are modified for all the entries,
    the value of this kernelopts variable is modified. But if the cmdline is
    modified for a single entry, then the kernelopts variable is expanded and
    the resulted modified value stored in the BLS snippet.
    
    The grubby tool expanded all the variables in the options field, but it's
    possible that some of these variables are managed by another tool. For
    example the tuned program adds a tuned variable that shouldn't be changed
    by the grubby tool. Otherwise it will mangle the tuned configuration set.
    
    To prevent this, let's make grubby to only expand the kernelopts variable.
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>