sysbuild: refactor image_config.cmake handling

Refactor image_config.cmake so that it is no longer sourced
unconditionally for all images. Instead image_config.cmake has been
split into BOOTLOADER_image_default.cmake and MAIN_image_default.cmake
and is set as property on the image.

This means the code in image_config.cmake can be split into dedicated
files which depends on the image type. Furthermore it allows sysbuild
modules to append extra config snippets to process for sysbuild kconfig
overwrite, or even remove the default snippet and have full control.

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