Prefer the default camera-source element if there is one or an element identified by an environment variable to a static list of possible elements which may not be appropriate for the target environment. Change-Id: I53816c949307953780f9046eb11e09effe059be0 Reviewed-by: John Brooks <john.brooks@dereferenced.net> Reviewed-by: Andy Nichols <andy.nichols@digia.com> Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>