kern: Add grub_debug_enabled()
    
    Add a grub_debug_enabled() helper function instead of open coding it.
    
    Signed-off-by: Peter Jones <pjones@redhat.com>
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
    Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>