Generate config check headers

Infrastructure to generate config check headers, to be included in
`${PROJECT}_config.c`.

This commit creates a module with the core logic, and support for three
generated files, to be included before the user config, just after the user
config, and after finalizing the user config.

This commit provides a basic `Checker` class. Future commits will add
derived classes for common cases such as options set in the wrong project.

This module is meant to be used in an executable script in each project,
containing the project-specific names to use and the list of options.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>