Targeting Mobile only in a windows universal app

To make your UWP app only target windows 10 mobile add the following to your app manifest

<Dependencies>

   <TargetDeviceFamily Name="Windows.Mobile" MinVersion="10.0.x.0" MaxVersionTested="10.0.y.0"/>
</Dependencies>

This MSDN article has more info
https://msdn.microsoft.com/en-us/library/windows/apps/dn986903.aspx