docs: kbuild: add references on Kconfig semantics

Add references to 1) a research paper which provides a definition of
Kconfig semantics, 2) the kismet tool, which checks for unmet direct
dependency bugs in Kconfig specifications.

Signed-off-by: Paul Gazzillo <paul@pgazz.com>
Signed-off-by: Necip Fazil Yildiran <fazilyildiran@gmail.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>