cmake: sysbuild: import image kconfig settings to image target

Load image kconfig setting into image target properties.
This allows sysbuild to evaluate and check image configuration as part
of CMake invocation.

sysbuild_get() is updated to support reading of CMake cache or Kconfig
settings for an image.

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