Draft: Add ostree-based image recipe to Apertis for RK3399

Adding an OSTree based image for RK3399 targets.

This MR includes a helper script to detect and mount the current ostree deploy dir. This is required to get the booloader files during the final image generation.

Note that other scripts are already doing this, so this helper script could be called from there, e.g.:

• https://gitlab.apertis.org/infrastructure/apertis-image-recipes/-/blob/apertis/v2022dev1/scripts/setup-uefi-bootloader.sh#L22
• https://gitlab.apertis.org/infrastructure/apertis-image-recipes/-/blob/apertis/v2022dev1/scripts/setup_rpi_boot.sh#L19

changed the description

changed target branch from apertis/v2022dev1 to apertis/v2022dev3