Support changing the username

Currently the have the hardcoded user named "user" (which is a bit confusing for new users). One thing that annoys me about the current mobile platforms is that I can't change the username so I have to remember what user to use when ssh-ing into the device.

Also for a proper linux installation the username shouldn't matter, I guess the only thing that depends on the username on the phone is some init scripts maybe, most hardcoded stuff is in the build chroots.