That's the easy bit. 16.04 flavour maker images came with a comprehensive ppa which needs updating too or converting to something else to achieve feature parity. (I do wonder how much of it is actually used, you don't see it written about.)
Then if you want to make official images you need to integrate the build into the various ubuntu packages that deal with this. There's probably at least three packages involved in that. Do you want a pre-installed image or a 'live' image? If it was me I would go with the latter, like omap4 used to do, see https://wiki.ubuntu.com/ARM/OmapDesktopInstall and http://old-releases.ubuntu.com/releases/quantal/ . That will require further updates to flash-kernel-installer or ubiquity. None of these come with any documentation so simple changes can take a long time if you've never done it before.