Join thread_move instead of killing it

Once we change the operational mode to STOPPED in the main thread, the
move thread should stop by itself, so we can wait for it to join.

Author: urfeex

resources/external_control.urscript

@@ -806,7 +806,7 @@ while control_mode > MODE_STOPPED:
 end
 
 textmsg("ExternalControl: Stopping communication and control")
-kill thread_move
+join thread_move
 kill thread_trajectory
 kill thread_script_commands
 stopj(STOPJ_ACCELERATION)