ci-license-scan: Add --fail-on-unknown for stuff not in target

Currently ci-license-scan fails if it can't detect the license of some files, even if are no licensing restrictions in the component that hosts the package.

This is annoying because it causes the scan to block pipelines where there's little value in adding the missing licensing information: for instance, while it is important on target, it is not particularly relevant everywhere else.

To avoid that, add a flag to define where failing to detect a license is problematic, otherwise make the command succeed and simply pass through the UNKNOWN entries in the licensing report.