fdt: Don't overwrite bloblist devicetree

When the devicetree comes from a bloblist, it is currently overwritten
by the appended one, if present. It should be preserved.

Adjust the logic to support this.

Fixes: 70fe2385 ("fdt: Allow the devicetree to come from a bloblist")

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