Add support for `b2c.quiet`

boot2container is a great tool, but it is IMO way too verbose. Looking at https://gitlab.freedesktop.org/bentiss/ci-templates/-/jobs/26990109 for instance and finding where the container is starting/ending is quite difficult.

I understand we need some verbose logs when things go wrong, but as a user, I don't care about them at all, I just care about my container commands.

So I am proposing for a new b2c.quiet option that would basically encapsulate in [collapsed]=true all of the initramfs outputs, except for the container. Having a set of information like "pulling the container", "setting up network" is perfectly fine as long as all the sections are collapsed.

While developing (or in the CI), not having the quiet option is definitely OK so it needs to be a user parameter.