cmake: dts: Use temporary file for dts.cmake

Uses a temporary file for dts output then uses CMake to copy to the
correct file if it has changed. This prevents a ping-pong issue when
sysbuild is used of configuring and building cycle when nothing has
changed and there is sysbuild code which loads in the devicetree
data from an image

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>