Various backports from v2022dev0 to v2021pre

Rebase everything from apertis/v2022dev0 up to 866cd577 with the exception of 8f99c173. Most of it has already been tested on the ApertisPro side, on top of v2021dev3 even.

• 56b7d3d5 README: Move the GitLab CI section at the end
• 80657135 README: Use the "GitLab CI/CD" spelling
• ce93a159 README: List the imx6qsabrelite u-boot installer image
• b41ae8ca README: Briefly document the GitLab CI/CD pipeline
• b7d70b14 gitlab-ci: Rework appending UPLOAD_ROOT to build-env.txt
• f9c1f0fa gitlab-ci: Set BUILD_ENV_OVERRIDE to customize the build
• e9adcf69 gitlab-ci: Capture RECIPES_URL in build-env.txt
• b06540ca gitlab-ci: Drop unused message var when building OSTree images
• f28b46f9 gitlab-ci: Fix warnings about multiple pipelines
• 28fb403c gitlab-ci: Tag more jobs as lightweight
• 4123edb5 Drop the apertis- prefix from the recipe names
• e61cf70a gitlab-ci: Make the APT mirror easier to configure
• c432a22f gitlab-ci: Auth to Docker registry when retrieving digest
• bab3178c Derive the keyring package name from the osname
• 64e46f9c rcar-gen3-uboot-extraction: Simplify default values
• 2e421512 rcar-gen3-uboot-extraction: Fail on unset variables
• 3b717735 rcar-gen3-uboot-extraction: Move under scripts/
• d859f832 gitlab-ci: Propagate the osname to the recipes
• 6864ef08 Drop .arcconfig
• 0db95f59 installer: Use the right osname for the local chdist config
• 2c75b4c8 setup_image_version: Always get the right osname
• dacef18e hostname: Set it dynamically using the osname
• ad89e5ab gitlab-ci: Interrupt pipelines if their branch is updated
• 7dbcc43d rcar-gen3-uboot-extraction: Simplify OSNAME default value handling
• 8831c4f1 gitlab-ci: Make the hawkBit server address easier to customize
• 7b1e6bb8 sample-image-production: Fix osname setting
• a4d3eb63 Add PWM modules into initramfs for armhf images
• af134283 Drop commented out packages
• 07084d31 Move to merged usr
• 2ba83025 ospack: target: Install flatpak on target by default
• 7dd4744d Drop extra modules in initramfs
• ad0e446b Use USB mass-storage automount on target and minimal images