grubby-bls: expand kernel options if these are environment variables
    
    The grub2 bootloader allows the BLS snippets fields to contain environment
    variables instead of constants. This is useful for example to define the
    kernel command line parameters only once in the grubenv file and set this
    variable in the BLS snippets.
    
    But this can be confusing for users when the information is printed by the
    grubby script, so expand the variables if they can be looked up in grubenv.
    
    Resolves: rhbz#1649785
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>