Add support for defining target for junction elements

• Add support for defining target for junction elements

  Add support for specifying target of a junction element to be a sub-sub-project as a configuration option. When such a target is defined, the junction element is not allowed to have any sources.

  This was discussed in the following mailing list thread: https://mail.gnome.org/archives/buildstream-list/2019-April/msg00025.html

• Add tests for specifying targets of junction elements

• NEWS: Add entry about junction targets