Fix up the partitioning for arm64 images so USB will work again

This code was using "-efi-boot-part", which works fine for booting but
creates images that d-i can't use: only the full disk or the ESP
partition would contain working filesystems, and d-i doesn't like that.

Instead, switch to appending the ESP as an extra partition on the end
of the image. This will take a small amount of extra space, but will
at least work reliably!