rust: Create zephyr-sys for low-level bindings

With a lot of work done by Mario Jaun <mario.jaun@zuehlke.com>.

Use the rust-bindgen tool to extract bindings from the Zephyr header files.
The tool is run with the exact configuration of the current build, and
therefore the bindings will match the current target.  The ifdefs and such
are not translated into the Rust code, and this must be generated live for
each build.

Signed-off-by: David Brown <david.brown@linaro.org>