Create Ubuntu 24.04 LTS images

Ubuntu 24.04 arrived 4/24. We will make x86_64 and aarch64 variants.

This image will use the new v4 ( @hibler ?) MBR, which will have a 64G root partition.

A new wrinkle for the aarch64 image is to see if we can achieve u-boot/UEFI dual-boot, as we did for BIOS/UEFI in the x86-64 case. This should not be a technical issue, since u-boot firmware knows where to look for the kernel/initrams, and the UEFI boot path goes through the efisyspart. If there is an issue, most likely it will be package conflicts between the grub-efi and u-boot packages, and/or the grub update hooks won't do the right thing in both places (this was the case for grub-pc-bin and grub-efi as I recall).