Specifier for the image name (as used in `Dependencies=`)

[NekkoDroid]

Currently there is no way to refer to the name of a given image when using mkosi.images, which is a minor problem when ImageId= is set as they will all default to the same name.

With a specifier for the name you can reduce the duplication where the image name is explicitly named and needs to be kept up-to-date when making changes.