kconfig: convert to SPDX License Identifier

All files in lxdialog/ are licensed under GPL-2.0+, and the rest are
under GPL-2.0. I added GPL-2.0 tags to test scripts in tests/.

Documentation/process/license-rules.rst does not suggest anything
about the flex/bison files. Because flex does not accept the C++
comment style at the very top of a file, I used the C style for
zconf.l, and so for zconf.y for consistency.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/kconfig/merge_config.sh

@@ -1,4 +1,6 @@
 #!/bin/sh
+# SPDX-License-Identifier: GPL-2.0
+#
 #  merge_config.sh - Takes a list of config fragment values, and merges
 #  them one by one. Provides warnings on overridden values, and specified
 #  values that did not make it to the resulting .config file (due to missed
@@ -10,15 +12,6 @@
 #
 #  Copyright (c) 2009-2010 Wind River Systems, Inc.
 #  Copyright 2011 Linaro
-#
-#  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.
 
 clean_up() {
 	rm -f $TMP_FILE