sandbox: Use os functions to read host device tree

At present we use U-Boot's filesystem layer to read the sandbox device tree,
but this is problematic since it relies on a temporary feauture added
there. Since we plan to implement proper block layer support for sandbox,
change this code to use the os layer functions instead. Also use the new
fdt_create_empty_tree() instead of our own code.

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