cmake: modules: extensions: Fix dts watch file processing

Fixes and simplifies the handling of how the dts watch file is
processed

Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>
Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
(cherry picked from commit 11c1f3de)