[U-Boot] RFC unified boot environment

[Dennis Gilmore]

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