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>

kconfig: tests: test if new symbols in choice are asked

If new choice values are added with new dependency, and they become
visible during user configuration, oldconfig should recognize them
as (NEW), and ask the user for choice.

This issue was fixed by commit 5d09598d488f ("kconfig: fix new choices
being skipped upon config update").

This is a subtle corner case. Add a test case to avoid breakage.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Ulf Magnusson <ulfalizer@gmail.com>