It is merged, but not included in the nightly builds. The reason is that we can either make a BIOS-compatible, or an UEFI-compatible image, but not both in the same image. Some extra work will be needed for that.
So if you want an UEFI image, you can use one of the unofficial ones (I don't have a link at hand), or build one yourself. Don't worry, once installed, you can update it in the normal way, so it is not a problem to start with an old-ish image.