[U-Boot] RFC unified boot environment

Hi All,

The attached patches build on the work for generic distro config, as well as Stephen Warrens implementation for tegra.

I have added a header to be included that will allow all boards to boot in the same way and I have ported over two boards to use it. One thought I have had is to move the environment into the same header as the generic config.

I am also working on writing a README.distro file to document everything that needs to be set for everything to just work. Along with information on how to best choose memory locations.

Dennis