samples: introduce 'with_mcuboot' sample to show sysbuild feature

This sample is intended to show users how they can create a sample which
has dedicated configuration files for extra images included in a build.

MCUboot is used as example on how its default can be adjusted and
MCUboot itself automatically be included in the build when using
sysbuild.

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