Update local aports location in 'init'

This addresses #246 (closed) by updating the local aports folder location every time pmbootstrap init is called. This location would change if an existing config file is in ~/.config and pmbootstrap's location has changed (e.g. when switching to a different fork in a different location on disk). With this commit, you just have to run pmbootstrap init and change (or accept) the existing config values and aports location is updated automatically.