[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.h

@@ -1,15 +1,3 @@
-/**
- * \addtogroup rime
- * @{
- */
-
-/**
- * \defgroup packetbuf Rime buffer management
- * @{
- *
- * The packetbuf module does Rime's buffer management.
- */
-
 /*
  * Copyright (c) 2006, Swedish Institute of Computer Science.
  * All rights reserved.
@@ -49,6 +37,18 @@
  *         Adam Dunkels <adam@sics.se>
  */
 
+/**
+ * \addtogroup rime
+ * @{
+ */
+
+/**
+ * \defgroup packetbuf Rime buffer management
+ * @{
+ *
+ * The packetbuf module does Rime's buffer management.
+ */
+
 #ifndef PACKETBUF_H_
 #define PACKETBUF_H_