[libc] Add stdbool.h standard header

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/stdbool.h

@@ -0,0 +1,10 @@
+#ifndef _STDBOOL_H
+#define _STDBOOL_H
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+#define bool _Bool
+#define true 1
+#define false 0
+
+#endif /* _STDBOOL_H */