kconfig: Add linting script

Add a cleaned-up version of a script I used to find a bunch of unused
symbols and some other Kconfig issues. It's set up to be run directly,
with few environment dependencies.

West is required, because the checks need to see Kconfig files and
source code from all modules.

Checks so far:

 - Symbols that can never be anything but n/empty

 - Symbols that look unused

 - menuconfig symbols with empty menus

 - Symbols only defined in Kconfig.defconfig files

See the help strings for the command-line flags for more information.

Some of these checks could probably be checked in CI later, though the
always-n and unused-symbol checks are a bit heuristic and might need a
lot of whitelisting.

Another reason I want to get this in is to have a clean standalone
reference for how to set up the environment for parsing the Kconfig
files. It's gotten trickier over time.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>