sysbuild: Add support for COMPILER_WARNINGS_AS_ERRORS

Adds support for the Kconfig option to enable compiler warnings
being treated as errors in all images that are build. This is a
sticky-set option which means enabling it in sysbuild will
enable it in all images, if it is then disabled in sysbuild it
will not be disabled in any of the images as a result and would
need to be manually unselected.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>