grubby-bls should only check if kernel exists and not if was installed
    
    When a new BLS entry is added, the script checks if the kernel image exists
    and also if it was installed from a rpm package. But the latter isn't really
    needed, it should be valid to just copy a kernel image and add a BLS entry.
    
    Resolves: rhbz#1634740
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>