of: let of_unflatten_dtb return the unflattened tree
In order to be able to handle multiple devicetrees, do not assume
the tree to be unflattened is the barebox internal one. Instead,
just return a pointer to it and assign the barebox internal root_node
external to the unflatten function.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>