Backports from v2022dev0 to v2021pre

Backport $BUILD_ID support and customizability from v2022dev0 as of c053666f to v2021pre.

• f112ff9d pkg-merge-upstream-to-downstreams: Fix targeting stable branches
• 61d50220 gitlab-ci: Inline the test jobs
• 05caabb5 gitlab-ci: Accept $BUILD_ID CI var to create additional tags
• 8b078ac7 gitlab-ci: Fix job dependencies
• 8591ac2d Drop .arcconfig
• 13daa3d9 Drop the apertis- prefix from the image folders
• 1f8be70b base: Rename the recipe to rootfs.yaml
• f8fe732e base: Rename the ospack file to a neutral "rootfs"
• 78d71b4a Drop the APT GPG key from images that get it via the base container
• d687bf07 Make the OS name configurable
• 3b1b4d84 Make the APT mirror easier to customize