scripts: zephyr_flash_debug: refactor into new "runner" package

Factor the classes which do the work into a new "runner" package. This
package has a core module where ZephyrBinaryRunner and common helpers
will live, and one file per subclass / runner front-end.

The top-level script, zephyr_flash_debug.py, still exists, but just
delegates its work to the core.

Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>