Due to how `disk-image` file is structured, it builds BIOS and UEFI
images at the same time. However, certain images (e.g., GCE images)
require only UEFI image to be built, BIOS image is being simply
discarded. This results in longer build times.
Splitting out `disk-image-uefi` would allow images to use it instead of
`disk-image` and thus avoid building unused BIOS images.
`disk-image` now depends on `disk-image-uefi` for backward
compatibility.
