Makefile: Use SPDX identifier

Change-Id: Ia05f2ecd31d0606dfe2bca843dbe6df2b8212a27
Signed-off-by: Jacob Garber <jgarber1@ualberta.ca>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/41598
Reviewed-by: HAOUAS Elyes <ehaouas@noos.fr>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/LICENSES/retained-copyrights.txt b/LICENSES/retained-copyrights.txt
index 6775d2e..671f37f 100644
--- a/LICENSES/retained-copyrights.txt
+++ b/LICENSES/retained-copyrights.txt
@@ -25,6 +25,8 @@
 Copyright (C) 2008 Advanced Micro Devices, Inc.
 Copyright (c) 2008, Google Inc.
 Copyright (C) 2008 Jordan Crouse <jordan@cosmicpenguin.net>
+Copyright (C) 2008 Uwe Hermann <uwe@hermann-uwe.de>
+Copyright (C) 2009-2010 coresystems GmbH
 Copyright (c) 2009-2012, Code Aurora Forum. All rights reserved.
 Copyright (c) 2010-2017, The Regents of the University of California
 Copyright (c) 2010, Code Aurora Forum. All rights reserved.
@@ -36,6 +38,7 @@
 Copyright (c) 2011 - 2014 The Linux Foundation. All rights reserved.
 Copyright (c) 2011-2019 The Linux Foundation. All rights reserved.
 Copyright (c) 2011, Google Inc.
+Copyright (C) 2011 secunet Security Networks AG
 Copyright (c) 2012 - 2013, 2015, 2019 The Linux Foundation.
 Copyright (c) 2012 - 2013, 2015 The Linux Foundation. All rights reserved.
 Copyright (c) 2012 - 2013 The Linux Foundation. All rights reserved.