firmware = "efi"
enables USB-boot from USB-disks or sticks that are prepared with UEFI-bootable boot loaders
firmware = "efi"
efi.legacyBoot.enabled = "true"
enables USB-boot with bootloaders like grub4dos and others - so yes - you can use bootsticks created in gthe XP-days
Note: this feature is neither documented nor supported - so have fun on your own risk