Fix some BLS snippets not being displayed in the GRUB menu
    
    There was an error in the logic that stored the parsed BLS snippets in the
    sorted linked list that is used to populate the GRUB boot menu entries.
    
    Also add a fix found by coverity scan about a possible undefined behaviour
    due grub_efi_status_t having the wrong type.
    
    Resolves: rhbz#1691232
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>