use CmakeLists scope to generate te XML file

- the only difference really is, that scope (the configuration.cmake) is already passed
  to px_process_params via the argument --scope. The Paths in --scope are evaluated w.r.t
  the path to src provided via the -s /--src-path argument.
- if no --scope is proveided. the Old scheme by simply walking the full --src-path directory
  is applied

CMakeLists.txt

@@ -364,7 +364,9 @@ px4_generate_messages(TARGET msg_gen
 	OS ${OS}
 	DEPENDS git_genmsg git_gencpp prebuild_targets
 	)
-px4_generate_parameters_xml(OUT parameters.xml BOARD ${BOARD})
+px4_generate_parameters_xml(OUT parameters.xml 
+                            BOARD ${BOARD}
+                            SCOPE ${PX4_SOURCE_DIR}/cmake/configs/${OS}_${BOARD}_${LABEL}.cmake)
 px4_generate_airframes_xml(OUT airframes.xml BOARD ${BOARD})
 add_custom_target(xml_gen
 	DEPENDS parameters.xml airframes.xml)