Nexus 5 not detected in Ubuntu 14.04
Do you have adb installed? Added the android rules and etc? I was in the same boat last night kind of, was trying to run a logcat. Google adb for whateverdistro you are using. Once I set up adb/android rules, my phone would show up via lsusb

Also, not sure if it matters, but do you have USB debugging turned on? 

I assume since you are trying to build an image, that you know the basics, but have you tried searching/posting on xda as well?