Add multiboot images

[cilki]

It should be possible to support multiboot images (multiple operating systems in one image) by running the regular build process on each template in parallel and then merging the results into a final image.

The trick will be how to create a new EFI partition based on the originals. Merging those partitions might be complicated, so it may be best to just preserve them and chainload from GRUB.

[cilki]

Note to myself later that this project may be helpful: https://www.rodsbooks.com/refind/.