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.

Signed-off-by: Ariel D'Alessandro <ariel.dalessandro@collabora.com>
Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>