This change refactors base OS sysext builds to use a separate build
script `build_library/sysext_prod_builder`, which is called from
`build_library/prod_image_util.sh` when `build_image` runs.
This allows for better separation of cleanup traps: prod image sysext
builds need its own trap / cleanup function for temporary build
directories and loopback mounts.
Prod sysext builds properly generate lincense and SBOM information, and
provide detailed file listings and disk space usage stats.
- SBOM / licenses JSON now include all packages of the
final image, i.e. a combined list of base image and all base OS
sysexts.
- Packages lists, files list and detailed files list include the sysext
squashfs files for the base image, and separate sections with files /
packages lists for each sysext.
- Disk usage contains both final disk image usage as well as usage of
each individual sysext squashfs.
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