[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/llsec/nullsec.h

@@ -1,16 +1,3 @@
-/**
- * \addtogroup llsec
- * @{
- */
-
-/**
- * \defgroup nullsec
- * 
- * Insecure link layer security driver.
- * 
- * @{
- */
-
 /*
  * Copyright (c) 2013, Hasso-Plattner-Institut.
  * All rights reserved.
@@ -50,6 +37,19 @@
  *         Konrad Krentz <konrad.krentz@gmail.com>
  */
 
+/**
+ * \addtogroup llsec
+ * @{
+ */
+
+/**
+ * \defgroup nullsec
+ * 
+ * Insecure link layer security driver.
+ * 
+ * @{
+ */
+
 #ifndef NULLSEC_H_
 #define NULLSEC_H_