Make the image devicetrees available to sysbuild for advanced use cases.
This is done by calling `zephyr_dt_import` on each image target. The DT
target properties are all expected to be prefixed with `DT_`, so there
should be no conflicts.
Thus, the DT contents of a given `<image>` can be retrieved using e.g.:
dt_nodelabel(<var> NODELABEL <label> TARGET <image>)
as noted in `extensions.cmake`.
Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>
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