sysbuild: support sysbuild/CMakeLists.txt as entry point for samples

This commit refactors sysbuild entry code by creating a CMake sysbuild
module for image processing and place sysbuild entry code in a
<app>/sysbuild/CMakeLists.txt file.

A template/CMakeLists.txt file will be use as template for applications
which doesn't provide their own entry file.

An application may create a sysbuild/CMakeLists.txt file.
The sysbuild/CMakeLists.txt file is similar in nature to the
toplevel CMakeLists.txt file but intended to used by sysbuild.
This allows application developers to adjust how an application is
built with sysbuild.

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