[LinuxBIOS] Add support for a 64-bit target for Serengeti Cheetah

[ron minnich]

I've had really bad luck with buildroot lately. It seems to have
passed some sort of event horizon of mass, and it is really easy to
drive yourself into  corner to the point that you have a broken build,
a broken bulid system, nothing compiles, and nothing works -- and your
every attempt at reconfiguration only makes it worse. I am not sure
what the future of buildroot will be.

So, I am happy that buildrom can do what it does.

ron