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/streamline_config.pl

@@ -1,7 +1,7 @@
 #!/usr/bin/env perl
+# SPDX-License-Identifier: GPL-2.0
 #
 # Copyright 2005-2009 - Steven Rostedt
-# Licensed under the terms of the GNU GPL License version 2
 #
 #  It's simple enough to figure out how this works.
 #  If not, then you can ask me at stripconfig@goodmis.org