Decouple `/etc/os-release` from `filesystem` package

[sighook]

Description:

Currently, the filesystem package ships /etc/os-release. This causes noise whenever we bump release metadata, since it forces a rebuild of filesystem even if nothing else changed.

Proposal:

• Move /etc/os-release into a dedicated os-release package.
• Version of os-release should track the actual Zeppe‑Lin release version (e.g. 1.2.2).
• Use package revision bumps for metadata‑only changes.
• Keep filesystem focused on directory layout and base paths, independent of release metadata.

Benefits:

• Cleaner release tagging workflow.
• Easier maintenance of rootfs and binpkgs.
• Clearer separation of concerns between base filesystem and release metadata.
• Reduces unnecessary rebuild noise.

[sighook]

Important: Zeppe-Lin Codebook needs to reflect that changes. Currently, it points to filesystem/os-release file in System Release Metadata section.