Add a main program so that we can run U-Boot.

Signed-off-by: Simon Glass <sjg@chromium.org>

We want to keep all OS-dependent code in once place, with a simple interface
to U-Boot. For now, this is that place.

Signed-off-by: Simon Glass <sjg@chromium.org>

These files are taken from the ARM board implementation and then reduced
to remove unneeded cruft.

Ideally we would work towards unifying arch/xxx/lib files, particularly
board.c.

Signed-off-by: Simon Glass <sjg@chromium.org>

This is an initial implementation with all functions defined but not working.

The lds file is very simple since we can mostly rely on the linker defaults.

Signed-off-by: Simon Glass <sjg@chromium.org>

We won't actually load an image with this architecture, but we still need to
define it.

Signed-off-by: Simon Glass <sjg@chromium.org>

This adds required header files for the sandbox architecture, and a basic
description of what sandbox is (README.sandbox).

Signed-off-by: Simon Glass <sjg@chromium.org>