Fix systemd-based OS customisation paths for Raspberry Pi OS Bookworm and newer.

[dianjixz]

Summary

Fix systemd-based OS customisation paths for Raspberry Pi OS Bookworm and newer.

Raspberry Pi OS Bookworm changed the firmware/boot partition mount point from /boot to /boot/firmware, so the previous systemd.run=/boot/firstrun.sh kernel command line no longer points to the generated firstrun.sh script on Bookworm+ images.

This PR keeps backward compatibility with Bullseye/Buster and older images by selecting the path based on the target OS release_date.

Changes

• Add centralized systemd firstrun path selection based on OS release date.
• Use /boot/firmware/firstrun.sh and /boot/firmware/cmdline.txt for images released on or after 2023-10-11.
• Keep existing /boot/firstrun.sh and /boot/cmdline.txt behavior for older images or when the release date is unavailable/invalid.
• Update both customisation write paths:
  • normal image write path in downloadthread
  • fastboot customisation path in fastbootflashthread
• Update CustomisationGenerator so firstrun cleanup paths are provided by the caller instead of being hard-coded to /boot.
• Add/update tests covering:
  • legacy /boot/firstrun.sh path selection
  • Bookworm+ /boot/firmware/firstrun.sh path selection
  • correct cmdline.txt cleanup path in generated firstrun.sh
  • fastboot cmdline append behavior for Bookworm+ paths

Background

Raspberry Pi OS Bookworm, released on 2023-10-11, mounts the boot/firmware partition at /boot/firmware instead of /boot.

Without this fix, systemd customisation on Bookworm+ images appends:

systemd.run=/boot/firstrun.sh

but the script is only available from the running OS under /boot/firmware/firstrun.sh, so first-run customisation does not execute correctly.

Compatibility

Older Raspberry Pi OS releases remain unchanged. If release_date is earlier than 2023-10-11, missing, or invalid, the existing /boot paths are still used.

Testing

• Ran static diff check:

git diff --check

• Started a CMake configure/test build after installing CMake, but dependency fetch/build was interrupted before completion.
• The updated tests are included for the customisation generator and fastboot command line behavior.