mboot64: First C output.

Can run C code and have that printf something.

Signed-off-by: Andrei Warkentin <andrey.warkentin@gmail.com>
bootm64: can interact with console.

...and run some basic commands. Whoo...

Signed-off-by: Andrei Warkentin <andrey.warkentin@gmail.com>
mboot64: Documentation.

I think I've found some public VE docs. They don't fill all the gaps,
but are better than reading nothing.

Signed-off-by: Andrei Warkentin <andrey.warkentin@gmail.com>