This updates the minimal supported version of CMake to 3.16, which in turn enables us to use more CMake features and have a cleaner build system.
This is the version that provides most new features and also the one we use in our latest docker image for CI.
check_kconfigs.py would only correctly handle "source", but not "rsource", "osource"
or "orsource".
Fixed any warnings detected after updating the test.