Support generating Unified System Images

USIs are fundamentally UKIs, but the whole rootfs is embedded as initrd and no /etc/initrd-release is present. IOW, it's a initrd that does not mark itself as so and never switch_root to somewhere else.

Since nowadays archiso usually just automatically copies itself to RAM, this shouldn't create too much churn. The result would be a single .efi file, that can be trivially dropped into the ESP for maintenance work, instead of having to figure out the right boot params to be able to chainload archiso.