cmake: initial sysbuild / multi image support

This is the initial commit with system build, sysbuild.

Using CMake as infrastructure together with the Zephyr sysbuild allows
us to support a convenient way of building a sample and allow for extra
images to be built as part of a larger system.

It uses Kconfig for configuration of image builds.
This allows for future extension with additional build images.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>