sysbuild: generate .config.sysbuild for controlling build settings

Sysbuild now generates a .config.sysbuild config file which specifies
settings controlled by sysbuild.
Any setting specified in this .config will overrule user provided
setting, and a warning will be raised if the sysbuild controlled value
is different from the value specified by the user.

This has the following benefits:
- Allow sysbuild to control any build setting without adjustments to
  the existing Kconfig tree
- Allow sysbuild to adjust settings based on knowledge regarding enabled
  images / bootloaders.
- Cleanup CMake code, as settings in sysbuild no longer needs to be
  propagated using CMake cache variables.

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