efi_loader: using EFI_UNSUPPORTED for private authenticated variables

Improve error message for UEFI SCT tests.

Signed-off-by: Weizhao Ouyang <o451686892@gmail.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>

lib/efi_loader/efi_variable.c

@@ -163,6 +163,7 @@ static efi_status_t efi_variable_authenticate(const u16 *variable,
 		break;
 	default:
 		/* TODO: support private authenticated variables */
+		ret = EFI_UNSUPPORTED;
 		goto err;
 	}