Document how to use Qt Configuration Tool with Yocto

Document how to create a feature configuration for Qt, export it for Boot2Qt and build a device image that incorporates a Qt built using that configuration.

Also document the ability to export and import feature configurations, which allows them to be transferred between people. Or reused locally in different builds.

See http://blog.qt.io/blog/2018/11/30/use-qt-lite-yocto/ for some source material.