Found by Steev - "," is not a safe delimiter to use in various places. Switch to "|" instead.
Shift the extra_image helper function from boot-x86 to common.sh to help. Call extra_image with || true for now on boot-arm64 so we don't fail builds straight away. Remove that || true later once we're happy things work.
Copy the buster dirs to bullseye as a start
Steve McIntyre