TF-A: Redefine true/false definitions

This patch redefines 'true' and 'false' definitions in
'include/lib/libc/stdbool.h' to fix defect reported by
MISRA C-2012 Rule 10.1
"The expression \"0\" of non-boolean essential type is
being interpreted as a boolean value for the operator \"? :\"."

Change-Id: Ie1b16e5826e5427cc272bd753e15d4d283e1ee4c
Signed-off-by: Alexei Fedorov <Alexei.Fedorov@arm.com>

include/lib/libc/stdbool.h

 /*
- * Copyright (c) 2018, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2018-2020, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -9,8 +9,8 @@
 
 #define bool	_Bool
 
-#define true	1
-#define false	0
+#define true	(0 < 1)
+#define false	(0 > 1)
 
 #define __bool_true_false_are_defined	1