Refactor Zephyr port to also be intermediate Zephyr manifest-module

[gregshue]

The MCUboot project is an application repository downstream of the Zephyr repository, but it does not support being built and configured from application modules that are further downstream. This requires clearly separating the project configuration portion of boot/zephyr/ from the reusable, configurable "application" service (i.e., subsystem).

This port refactoring should bring alignment with the Zephyr configuration, build, and verification solutions to support future enablement of all current and anticipated functionality.