sysbuild: populate build_info with list of images

Populate the build_info.yml created by sysbuild CMake with list of
images and write the build info file afterwards.

This allows users and external tools to examine which images was
included in the build even if one or more of those images fails CMake
configure or generate stage.

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