Use ipxe.efi under UEFI (#927783)
Hi, this merge request addresses bugs.debian.org/927783. Specifically, currently iPXE configures GRUB to load ipxe.lkrn under UEFI, which is inappropriate, and hangs the computer. The patch configures GRUB to load ipxe.efi under UEFI.
Additionally, it saves the default entry, so that the GRUB iPXE entry is consistent with all the other entries.