actuator_motors.msg: add reversible flags & implement in mixer_module

msg/actuator_motors.msg

@@ -2,6 +2,8 @@
 uint64 timestamp			# time since system start (microseconds)
 uint64 timestamp_sample	    # the timestamp the data this control response is based on was sampled
 
+uint16 reversible_flags     # bitset which motors are configured to be reversible
+
 uint8 NUM_CONTROLS = 8
 float32[8] control # range: [-1, 1], where 1 means maximum positive thrust,
                    # -1 maximum negative (if not supported by the output, <0 maps to NaN),