core: moved definition of KERNEL_PID_NULL

kernel_pid_t is defined in kernel_types.h, thus it makes sense to move
the macro for an invalid kernel pid there, too.

core/include/kernel.h

@@ -67,12 +67,6 @@
 #endif
 
 /* ------------------------------------------------------------------------- */
-/**
- * @def PID_NULL
- * @brief Identifier to detect an invalid PID
- */
-#define KERNEL_PID_NULL -1
-
 /**
  * @def PRIORITY_MIN
  * @brief Least priority a thread can have

core/include/kernel_types.h

@@ -4,6 +4,12 @@
 #include <stdint.h>
 #include <inttypes.h>
 
+/**
+ * @def KERNEL_PID_NULL
+ * @brief Identifier to detect an invalid PID
+ */
+#define KERNEL_PID_NULL -1
+
 /**
  * Macro for printing formatter
  */