Harmattan: Introduce new condition for auto-generation of manifest file.

In addition to using an empty file as in 2.3, the user can now also put
a magic string at the start of the manifest file. This way, we can use
this as the default while also documenting it in-place.

Task-number: https://projects.maemo.org/bugzilla/show_bug.cgi?id=284797


Change-Id: I375a73a5d2e9d3e0973d97d81e0110e788d0159a
Reviewed-by: Eike Ziller <eike.ziller@nokia.com>