cmake: support MCUBoot bootloader natively with SYSBuild

This commit adds CMake and Kconfig files needed to build MCUboot as
an extra image using SYSBuild.

Building an application with MCUBoot using SYSBuild allows users to
build both images using a single build.

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