Fix grub-mkimage --append-signature-size
    
    0179-Add-suport-for-signing-grub-with-an-appended-signatu.patch was
    written in Jun of 2020, before support for .sbat went upstream.  It adds
    a command line option "--append-signature-size" to grub-mkimage with the
    short argument '-s'.
    
    Unfortunately, .sbat support also uses that short argument, and as a
    result, trying to use "grub-mkimage --append-signature-size" on ppc64le
    (where we need it) fails due to argument.sbat being set on a non-EFI
    platform.
    
    This patch makes the --append-signature-size use 'S' instead of 's'.
    
    Related: rhbz#1951104
    Signed-off-by: Peter Jones <pjones@redhat.com>