[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.c

@@ -1,8 +1,3 @@
-/**
- * \addtogroup nullsec
- * @{
- */
-
 /*
  * Copyright (c) 2013, Hasso-Plattner-Institut.
  * All rights reserved.
@@ -42,6 +37,11 @@
  *         Konrad Krentz <konrad.krentz@gmail.com>
  */
 
+/**
+ * \addtogroup nullsec
+ * @{
+ */
+
 #include "net/llsec/nullsec.h"
 #include "net/mac/frame802154.h"
 #include "net/netstack.h"