Improve support for platforms without stddef.h. NULL definition should now be provided by architecture port file atomport.h, which in most cases can just include stddef.h.

kernel/atomkernel.c

@@ -144,7 +144,6 @@
  */
 
 
-#include <stddef.h>
 #include "atom.h"
 
 

kernel/atomport-template.h

@@ -34,6 +34,12 @@
 /* Required number of system ticks per second (normally 100 for 10ms tick) */
 #define SYSTEM_TICKS_PER_SEC            100
 
+/**
+ * Definition of NULL.
+ * If stddef.h is available on the platform it is simplest to include it
+ * from this header, otherwise define below.
+ */
+#define NULL      ((void *)(0))
 
 /**
  * Architecture-specific types.