Overwriting upstart alias as folder under overlay

When trying to add media-hub.conf as overlay, overlay/xdg-config-dirs/upstart alias is replaced by overlay/usr/upstart from "snapcraft build", which blocks needed upstarts starting from unity8-session.

- previous
overlay
├── legacy-exec
├── snappyenv
├── usr
│   └── share
│   └── icons
│   └── default
│   └── index.theme -> ../DMZ-White/cursor.theme
└── xdg-config-dirs
    └── upstart -> ../usr/share/upstart/sessions

- to add media-hub.conf
overlay
├── legacy-exec
├── snappyenv
├── usr
│   └── share
│   ├── icons
│   │   └── default
│   │   └── index.theme -> ../DMZ-White/cursor.theme
│   └── upstart
│   └── sessions
│   └── media-hub.conf
└── xdg-config-dirs
    └── upstart -> ../usr/share/upstart/sessions

WIP branch with workaround:
https://code.launchpad.net/~chunsang/unity8-session-snap/media-hub-fix

NOTE: not sure if it's the right place to report this here, please comment if someone is aware of better place to move this.