Bug 1096898 - [Usage] Make Usage app work in Mulet

> Ok, my fault that I didn't make it clear, DEBUG=1 make doesn't create app package for device and doesn't use webapp-optimize.js at all (@julienw please correct me if I'm wrong). 

Mmm actually I think it's always performed. see https://github.com/mozilla-b2g/gaia/blob/master/build/webapp-optimize.js#L83. Which triggers this bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1099185 :)