combined system image artifact size is slowly growing out of hand

Currently, each (gzipped) system image is around 1.3 GB in size. There are currently 7 images with a combined artifact size of 9.35 GB. Pipeline artifacts are kept for a week before they are auto-deleted (if they do not belong to the latest commit in a branch) which means that right now 51.5 GB of artifacts are stored for the reform-system-image repo.

This situation will get worse when

• more platforms are added (reform next joining pocket reform and reform 2 soon)
• more SoMs are added (rk3588 joining imx8mq, imx8mplus, a311d and ls1028a soon)
• there is a week of heavy development with more than usual pipeline runs

Possible solutions:

• keep artifacts less long
• manually or automatically prune artifacts on demand
• prune stale branches
• limit the size of the system image matrix by only offering support for a subset of SoMs per platform
• sunsetting old or deprecated SoMs
• serve images by a webserver which concatenates the /boot with the root partition on demand
• offer multiple /boot images and a single root image and instruct the user to just do two dd operations
• dd them together before flashing them like done for Debian installer sd-card images: http://ftp.debian.org/debian/dists/bookworm/main/installer-arm64/current/images/netboot/SD-card-images/
• offer a single image with all device trees but without u-boot (can be flashed on-demand if there is no u-boot on emmc): !104 (closed)
• reduce size by reform-handbook by more than 200 MB: reform-handbook!16 (merged)
• compress with xz instead of gzip (1.3 GB -> 900 MB)
• share u-boot at least across the same board (even when put into different platforms)
• a common /boot partition could be achieved if:
  • we give up on old u-boot on imx8mq which does not support extlinux.conf
  • all platforms have a u-boot which sets $fdtfile (stock pocket imx8mp u-boot does not)
  • /boot/flash.bin would be put elsewhere