fdt: Allow the devicetree to come from a bloblist

Standard passage provides for a bloblist to be passed from one firmware
phase to the next. That can be used to pass the devicetree along as well.
Add an option to support this.

Tests for this will be added as part of the Universal Payload work.

Series-changes: 2
- No changes as it still seems unclear what should be done

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