efi_loader: bump UEFI specification number to 2.8

We are implementing UEFI variable RuntimeServicesSupported and set the
unimplemented runtime functions return EFI_UNSUPPORTED as described in UEFI
specification 2.8. So let's also advertise this specification version in
our system table.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Alexander Graf <agraf@csgraf.de>