copy target dtb to a temp directory for qcom and sprd dt.img types

The dtbTool and dtbTool-sprd programs require the dtb directory as an argument. These programs tend to combine multiple devicetrees. We can't determine for whether the directory has devicetree blobs for a single device, or multiple devices.

It's safer to copy the target dtb over to a temporary directory, and pass that directory as an argument to dtbTool.