Add arm64 image generation and allow for independent image generation

[nickshanks347]

This merge does two things:

1. Allows for independent image generation for ISO/IMG
2. Generate arm64 ISO/IMG files

ISO/IMG are only generated when generate_disks_legacy, generate_disks_arm and generate_disks_efi are set to true as well as generate_disks_hybrid. I believe any image should be able to be generated independently without having to have all of them generated. The code looks at the variables set and addresses this issue by explicitly declaring dependencies

The second change adds arm64 ISO/IMG generation. With an increasing number of desktops/SBCs using arm64, images for this architecture would be useful.

May be worth adding in separate images for legacy and EFI so image generation times can be cut down.

[antonym]

Thanks!