Adds a new experimental Kconfig option to sysbuild which allows
generating a variant image to use with MCUboot in direct-xip mode.
It also includes base sysbuild infrastructure for creating
additional variant images in future (including out-of-tree),
whereby they use the base configuration of an existing image but
modify it, without creating the default zephyr extra targets for
images (e.g. menuconfig) to prevent changing configuration that
should not be changed.
Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
8312b20ca9