Rework TRACEF format, handle empty varargs (GNU extension)

include/ucore/ucore_utils.h

@@ -10,9 +10,10 @@
     } while (0)
 
 #ifdef DEBUG
-    #define TRACEF(fmt, ...)\
+    #define TRACEF(fmt, args...)
     do { \
-        fprintf(stderr, "%s:%s(%d)" fmt, __FILE__, __FUNCTION__, __LINE,__VA_ARGS__);\
+        fprintf(stdout, "%s:%d (%s)\t" fmt, __FILE__, __LINE__,  __FUNCTION__,  ##args);\
+        fflush(stdout);\
     } while(0)
 #else
     #define TRACEF(fmt, ...)