px4fmu: added "mode_pwm4" startup mode

this is the default mode ArduPilot uses, and by setting it in the init
script we avoid any pin activity on the two GPIO pins during boot

src/drivers/px4fmu/fmu.cpp

@@ -1529,6 +1529,7 @@ enum PortMode {
 	PORT_GPIO_AND_SERIAL,
 	PORT_PWM_AND_SERIAL,
 	PORT_PWM_AND_GPIO,
+	PORT_PWM4,
 };
 
 PortMode g_port_mode;
@@ -1564,6 +1565,13 @@ fmu_new_mode(PortMode new_mode)
 #endif
 		break;
 
+#if defined(CONFIG_ARCH_BOARD_PX4FMU_V2)
+	case PORT_PWM4:
+		/* select 4-pin PWM mode */
+		servo_mode = PX4FMU::MODE_4PWM;
+		break;
+#endif
+
 		/* mixed modes supported on v1 board only */
 #if defined(CONFIG_ARCH_BOARD_PX4FMU_V1)
 
@@ -1836,6 +1844,10 @@ fmu_main(int argc, char *argv[])
 	} else if (!strcmp(verb, "mode_pwm")) {
 		new_mode = PORT_FULL_PWM;
 
+#if defined(CONFIG_ARCH_BOARD_PX4FMU_V2)
+	} else if (!strcmp(verb, "mode_pwm4")) {
+		new_mode = PORT_PWM4;
+#endif
 #if defined(CONFIG_ARCH_BOARD_PX4FMU_V1)
 
 	} else if (!strcmp(verb, "mode_serial")) {