Add Kconfig preprocessor

Implement the Kconfig preprocessor described in
https://github.com/torvalds/linux/blob/master/Documentation/kbuild/kconfig-macro-language.txt
(which is now in linux-next and will appear in Linux 4.18).

A new Kconfig.variables property holds all the preprocessor variables so
that they can be inspected programmatically. Preprocessor variables are
represented by a new Variable class.

With the preprocessor, environment variables are referenced with $(FOO)
instead of $FOO. For backwards compatibility, $FOO is accepted as well
for now (and leaves "$FOO" as-is if FOO doesn't exist). The $FOO syntax
might be dropped at some point in the future (together with a major
version increase). It should be supported for a few months at least.

Some internals were cleaned up too, mostly related to parsing. Some
outdated documentation was fixed as well.

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