Fix/Update copyright notices; Add SPDX annotation

Let's fix some erroneous/accidential references to wrong license,
update copyright information where applicable and introduce a
SPDX-License-Identifier to all files.

Change-Id: I39af26c6aaaf5c926966391f6565fc5936be21af
diff --git a/include/osmocom/gsm/oap.h b/include/osmocom/gsm/oap.h
index ce1e151..f33cb50 100644
--- a/include/osmocom/gsm/oap.h
+++ b/include/osmocom/gsm/oap.h
@@ -8,10 +8,11 @@
  *
  *  \file oap.h */
 /*
- * (C) 2015-2016 by sysmocom s.f.m.c. GmbH
+ * (C) 2015-2016 by sysmocom - s.f.m.c. GmbH
+ * Author: Neels Hofmeyr
  * All Rights Reserved
  *
- * Author: Neels Hofmeyr
+ * SPDX-License-Identifier: GPL-2.0+
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by