[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/net/packetbuf.c

@@ -1,8 +1,3 @@
-/**
- * \addtogroup packetbuf
- * @{
- */
-
 /*
  * Copyright (c) 2006, Swedish Institute of Computer Science.
  * All rights reserved.
@@ -42,6 +37,11 @@
  *         Adam Dunkels <adam@sics.se>
  */
 
+/**
+ * \addtogroup packetbuf
+ * @{
+ */
+
 #include <string.h>
 
 #include "contiki-net.h"