add copyright notice

The original board startup script is provided by Atmel.
It has been modified to handle application or DFU booting.
The copyright notice has been updated to reflect this change.
diff --git a/firmware/libboard/common/source/board_cstartup_gnu.c b/firmware/libboard/common/source/board_cstartup_gnu.c
index ce60b09..775f325 100644
--- a/firmware/libboard/common/source/board_cstartup_gnu.c
+++ b/firmware/libboard/common/source/board_cstartup_gnu.c
@@ -2,6 +2,7 @@
  *         ATMEL Microcontroller Software Support

  * ----------------------------------------------------------------------------

  * Copyright (c) 2010, Atmel Corporation

+ * Copyright (C) 2017, Harald Welte <laforge@gnumonks.org>

  *

  * All rights reserved.

  *