This change makes ironic building UEFI-only bootable
ISO image (when being asked to build a UEFI-bootable image)
rather than building a hybrid BIOS/UEFI-bootable ISO.
The motivation behind this change is to make node boot more
predicable in the sense that if the user asks for UEFI boot,
the node would either boot over UEFI or fail.
On top of that, the ISO image slims down a tad bit and syslinux
boot loader presence on the conductor machine is not a
requirement any more.
Change-Id: If7828f72724cc7532f97cab758a29a9e8345850e
Story: 1526753
Task: 28368
4008c9bfa4