refactor px4flow: use driver base class

ROMFS/cannode/init.d/rcS

@@ -97,10 +97,10 @@ unset BOARD_RC_SENSORS
 #
 sh /etc/init.d/rc.serial
 
-# Check for flow sensor, launched as a background task to scan
+# Check for flow sensor
 if param compare SENS_EN_PX4FLOW 1
 then
-	px4flow start &
+	px4flow start -X
 fi
 
 uavcannode start

ROMFS/px4fmu_common/init.d/rcS

@@ -467,10 +467,10 @@ else
 		vmount start
 	fi
 
-	# Check for flow sensor, launched as a background task to scan
+	# Check for flow sensor
 	if param compare SENS_EN_PX4FLOW 1
 	then
-		px4flow start &
+		px4flow start -X
 	fi
 
 	# Blacksheep telemetry