Yocto: Allow users to enable/disable qt features in custom projects

Qt modules have a feature system to enable or disable features. I.e. you can compile qtdeclarative without networking by calling inside qtdeclarative

  qmake -- -disable-feature-qml-network

Qt's Yocto recipes should directly support enabling and disabling features for custom projects, i.e. by evaluating a 'features' variable that can be set in a .bbappend file.