docs: generate documentation for Kconfig options

docs: generate documentation for Kconfig options

This adds a simple script which parses Kconfig files using Kconfiglib
and emits ReST document with the list of all options. For each option
a link target is generated, to make it possible to link to any Kconfig
option from the rest of the documentation.
Since Kconfiglib is not on PyPI, the latest version (45f87b9d) is
bundled into the docs directory.