Fix HDMI Video and refactor device trees

This pull request addresses fixing the video part of the HDMI interface for the parallella-hdmi machine, as well as refactoring the device trees used for both the HDMI and non-HDMI machines.

In addition to the device trees provided by the layer the linux-parallella kernel recipe is now setup to generate the device trees from the kernel sources. These device trees are generated with the prefix of 'uImage-*' and output to the deploy/images/ directory for the machine.