Made it explicit and clear. User may define as many file system images as he/she needs. Every image is to have its own uri, format, container and other metadata. Currently, it is possible to mix different image formats. For example, image for a root file system might be an ext4 fs image while /var image - TAR archive. Change-Id: Ib957ff62e3613fed1e2826f322a056be7803fa48