PWM automatic trigger system (ATS) for parachutes (#13726)

* parameter and logic to commander for triggering failsafe from external automatic trigger system.
* logic to startup script for enabling ATS. Added uORB publishing to pwm_input module.
* Refactored out CDev usage from pwm_input and ll40ls. Refactored out ll40ls specifics from pwm_input and cleaned up dead code.

ROMFS/px4fmu_common/init.d/rc.sensors

@@ -58,6 +58,12 @@ then
 	fi
 fi
 
+# External automatic trigger system
+if param compare FD_EXT_ATS_EN 1
+then
+	pwm_input start
+fi
+
 # Lidar-Lite on I2C
 if param compare -s SENS_EN_LL40LS 2
 then