[airborne] std.h: define NULL if not already done

sw/include/std.h

@@ -61,6 +61,14 @@
 #define TRUE (!FALSE)
 #endif
 
+#ifndef NULL
+#ifdef __cplusplus
+#define NULL 0
+#else
+#define NULL ((void *)0)
+#endif
+#endif
+
 /* Boolean values */
 typedef uint8_t bool_t;
 
@@ -156,7 +164,7 @@ typedef uint8_t unit_t;
 
 #define RunXTimesEvery(_jumpstart, _prescaler, _interval, _xtimes, _code) {		\
   static uint16_t prescaler = _jumpstart;			\
-  static uint16_t xtimes = 0;		         	\
+  static uint16_t xtimes = 0;                   \
   prescaler++;					\
   if (prescaler >= _prescaler + _interval*xtimes && xtimes < _xtimes) {			\
     _code;						\