kernel: profile system when SYSTEM_PROFILER is defined.

* This enables a mechanism to profile almost the complete boot process
  (starting with main2()), if SYSTEM_PROFILER is defined to 1.
* You can access the profiling data using "profile -r".

headers/private/kernel/system_profiler.h

@@ -9,15 +9,19 @@
 
 #include <OS.h>
 
+#include "kernel_debug_config.h"
+
 
 struct system_profiler_parameters;
 
 
 __BEGIN_DECLS
 
+#if SYSTEM_PROFILER
 status_t start_system_profiler(size_t areaSize, uint32 stackDepth,
 			bigtime_t interval);
 void stop_system_profiler();
+#endif
 
 status_t _user_system_profiler_start(
 			struct system_profiler_parameters* parameters);