fix(ubuntu): Fix broken Ubuntu bootenvs.

The Ubuntu bootenvs were referring to AMD64 EFI bootloaders that only knew how to boot either via USB key or cdrom, not the network. In the absence of working network enabled UEFI secure boot bootloaders on the install dvds, the best course of action is to fall back to ipxe when booting via UEFi, and accept that Secure Boot requires extra work to enable for Ubuntu and Debian.