parts: Trust: better warnings management

Allow fine-tuned management of trust warnings,
the user is now able to disable specific warnings
instead of blocking everything.

Also show warnings (if any) when onBoarding is completed

Change-Id: I463279a8d99c1eeb2be6c0eeaf2e53e8f97ac160
Signed-off-by: Joey <joey@lineageos.org>