[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.

apps/serial-shell/serial-shell.c

@@ -1,8 +1,3 @@
-/**
- * \addtogroup shell
- * @{
- */
-
 /*
  * Copyright (c) 2008, Swedish Institute of Computer Science.
  * All rights reserved.
@@ -41,6 +36,11 @@
  *         Adam Dunkels <adam@sics.se>
  */
 
+/**
+ * \addtogroup shell
+ * @{
+ */
+
 #include "contiki.h"
 #include "shell.h"