Intel® Joule™ compute module for Robotics has preliminary instructions that show how to install Ubuntu. I've only read it quickly but it looks like its a standard install, with a hack to reformat/repartition the eMMC,
There's more info and some QA at Re: Installing Ubuntu on Intel Joule 570x.
Note this is an install of Ubuntu 16.04 Desktop, not the Ubuntu Core, aka Snappy.
BTW, if you go this route, I recommend installing xubuntu. The drivers in this version of Ubuntu are not yet optimized for the Joule, so things like compiz take more of the CPU than they should. In my opinion though xubuntu is a better UI anyhow
The process is also not has hard as it looks, and if you ever want to go back to the standard Linux, that is still possible without too much trouble.
To install Ubuntu Core, there is now a release available: look at Intel Joule | Ubuntu developer portal . That page also documents how to install desktop Ubuntu (which I have already successfully installed ROS Kinetic under and run a SLAM navigation stack on) but there are couple of issues in the docs, mentioned in the thread at Installing Ubuntu on Intel Joule 570x. However, read this second thread with care since the it also documents a now-obsolete hacky way to install Ubuntu that is no longer recommended. Now that there is an official release from Canonical with the proper drivers, that is the preferred installation.