yocto-docs: Add SPDX headers in scripts and Makefile

[Yocto #13873]

(From yocto-docs rev: b87c540a1a6c0dce4c6010b89d9831442e0e7f70)

Signed-off-by: Armin Kuster <akuster808@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/tools/update-documentation-conf

@@ -1,23 +1,13 @@
 #!/usr/bin/env python
-
+#
+# SPDX-License-Identifier: GPL-2.0-only
+#
 # documentation.conf update script
 #
 # Author: Paul Eggleton <paul.eggleton@linux.intel.com>
 #
 # Copyright (C) 2015 Intel Corporation
 #
-# This program is free software; you can redistribute it and/or modify
-# it under the terms of the GNU General Public License version 2 as
-# published by the Free Software Foundation.
-#
-# This program is distributed in the hope that it will be useful,
-# but WITHOUT ANY WARRANTY; without even the implied warranty of
-# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-# GNU General Public License for more details.
-#
-# You should have received a copy of the GNU General Public License along
-# with this program; if not, write to the Free Software Foundation, Inc.,
-# 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 
 
 import sys