Improve documentation for Embedded Linux

In Qt 4 the documentation had information on OpenGL, DirectFB, etc. usage on Embedded Linux platforms. In Qt 5 this is all gone since QWS and Qt for Embedded Linux does not exist anymore. Platforms like eglfs and linuxfb would deserve some documentation nonetheless, especially since these feature a number of specific settings and limitations that do not usually apply to desktop platforms.

The wiki page at http://qt-project.org/wiki/i.MX-6 can be used as a starting point for the topics that need to be described.