ADD support for freescale i.MX6

Hi 

Here is a PR (split as 2 commits to ease reviewing) which adds support for the Freescale i.MX6 SystemOnChip.
I initiated this work a little more than one year ago in this [thread](http://forum.xbmc.org/showthread.php?tid=161793). Then, to ease collaboration, this porting effort moved to github [repo](https://github.com/xbmc-imx6/xbmc) when Chris contributed the rendering through vivante extensions.

This result is a collaborative work and I would like to thank especially @koying, @smallint and @warped-rudi who helped a lot with this development. Also maintainers of Geexbox, archlinux arm and openelec helped a lot.

This code is known to work on all i.MX6 variants and has been run successfully on a large number of boards/devices.

Sorry if everything is not ready for inclusion : I am fully available to take into account all your remarks or to provide explanations to enable this merge.

Stéphan
should be split into its own commit if not directly related to MX6