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

cmake/nuttx/px4_impl_nuttx.cmake

@@ -435,7 +435,7 @@ function(px4_nuttx_add_romfs)
 			--obj romfs.o
 			--var romfs_img
 			--bin romfs.bin
-		DEPENDS ${romfs_src_files} ${extras}
+		DEPENDS ${romfs_src_files} ${extras} ${optional_files}
 		WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
 		)
 	add_library(${OUT} STATIC romfs.o)