Leave some pins available for camera trigger GPIO.

ROMFS/px4fmu_common/init.d/rcS

@@ -298,6 +298,17 @@ then
 		set FMU_MODE gpio_serial
 	fi
 
+	if param greater TRIG_MODE 0
+	then
+		if [ $PWM_OUT == 1234 ]
+		then
+			if ver hwcmp PX4FMU_V4
+			then
+				set FMU_MODE pwm4
+			fi
+		fi
+	fi
+
 	if [ $HIL == yes ]
 	then
 		set OUTPUT_MODE hil