PCEngines Alix Support

I decided to make this a pull request into your ag150 branch since I don't think you want it in master yet.   Anyway, things work pretty well.   

This implements support for the PCEngines Alix series (specifically tested on the Alix 2d2 and 2d13).    GPIO, LEDs and networking have been tested to work properly.

I added support by adding a new configs/alix_defconfig and boards/alix directory.    Ideally nerves would have a way to reuse most of what is in the boards directory since many of my changes were were just changing ag150 to alix in configuration for instance.    I realize multi-board support is not something you've been working on.