[doc] Give files a common structure by placing license first (partial)

Followed by doxyen file and group tags.
This patch is only a first attempt to make provide a clean structure,
many more files require rework, too.

core/sys/stimer.c

@@ -1,15 +1,3 @@
-/**
- * \addtogroup stimer
- * @{
- */
-
-/**
- * \file
- * Timer of seconds library implementation.
- * \author
- * Adam Dunkels <adam@sics.se>, Nicolas Tsiftes <nvt@sics.se>
- */
-
 /*
  * Copyright (c) 2004, 2008, Swedish Institute of Computer Science.
  * All rights reserved.
@@ -44,6 +32,18 @@
  *
  */
 
+/**
+ * \file
+ * Timer of seconds library implementation.
+ * \author
+ * Adam Dunkels <adam@sics.se>, Nicolas Tsiftes <nvt@sics.se>
+ */
+
+/**
+ * \addtogroup stimer
+ * @{
+ */
+
 #include "contiki-conf.h"
 #include "sys/clock.h"
 #include "sys/stimer.h"