minikconfig: add parser skeleton

minikconfig: add parser skeleton

This implements a scanner and recursive descent parser for Kconfig-like
configuration files.  The only "action" of the parser is for now to
detect undefined variables and process include files.

The main differences between Kconfig and this are:

* only the "bool" type is supported

* variables can only be defined once

* choices are not supported (but they could be added as syntactic
sugar for multiple Boolean values)

* menus and other graphical concepts (prompts, help text) are not
supported

* assignments ("CONFIG_FOO=y", "CONFIG_FOO=n") are parsed as part
of the Kconfig language, not as a separate file.

The idea was originally by Ákos Kovács, but I could not find his
implementation so I had to redo it.

Signed-off-by: Paolo Bonzini <[email protected]>
Message-Id: <20190123065618[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>