hob: only add new layer's to the UI

hob: only add new layer's to the UI

If a user added the same layer more than once the underlying bblayers.conf
file would not have multiple entries but the UI would show an entry for
the layer for each time it was added. This patch changes the Configurator
to return None values if the layer wasn't added to bblayers.conf and the
LayerEditor to only update the UI if non-None values are returned.

Fixes [YOCTO #1275]

(Bitbake rev: 9c59baa4da3c863ebc412a56ffd8dbd7a8ffeb60)

Signed-off-by: Joshua Lock <josh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>