Copter: implement 'air-mode' for copter

this implements what betaflight calls 'air-mode'. This gives attitude
control when throttle is at zero, allowing for zero-throttle maneuvers,
plus keeping the copter level on the ground.

This was already implemented if an interlock switch was setup, but it
should also work with an arming switch. If using an arming switch then
throttle should not be considered zero as long as the arming switch
hasn't gone low.

ArduCopter/Copter.h

@@ -333,6 +333,7 @@ private:
             uint8_t initialised_params      : 1; // 25      // true when the all parameters have been initialised. we cannot send parameters to the GCS until this is done
             uint8_t compass_init_location   : 1; // 26      // true when the compass's initial location has been set
             uint8_t rc_override_enable      : 1; // 27      // aux switch rc_override is allowed
+            uint8_t armed_with_switch       : 1; // 28      // we armed using a arming switch
         };
         uint32_t value;
     } ap_t;

ArduCopter/motors.cpp

@@ -230,6 +230,9 @@ bool Copter::init_arm_motors(bool arming_from_gcs)
     // Start the arming delay
     ap.in_arming_delay = true;
 
+    // assumed armed without a arming, switch. Overridden in switches.cpp
+    ap.armed_with_switch = false;
+    
     // return success
     return true;
 }

ArduCopter/radio.cpp

@@ -178,7 +178,9 @@ void Copter::set_throttle_zero_flag(int16_t throttle_control)
     // if not using throttle interlock and non-zero throttle and not E-stopped,
     // or using motor interlock and it's enabled, then motors are running, 
     // and we are flying. Immediately set as non-zero
-    if ((!ap.using_interlock && (throttle_control > 0) && !ap.motor_emergency_stop) || (ap.using_interlock && motors->get_interlock())) {
+    if ((!ap.using_interlock && (throttle_control > 0) && !ap.motor_emergency_stop) ||
+        (ap.using_interlock && motors->get_interlock()) ||
+        ap.armed_with_switch) {
         last_nonzero_throttle_ms = tnow_ms;
         ap.throttle_zero = false;
     } else if (tnow_ms - last_nonzero_throttle_ms > THROTTLE_ZERO_DEBOUNCE_TIME_MS) {

ArduCopter/switches.cpp

@@ -638,6 +638,8 @@ void Copter::do_aux_switch_function(int8_t ch_function, uint8_t ch_flag)
             switch (ch_flag) {
             case AUX_SWITCH_HIGH:
                 init_arm_motors(false);
+                // remember that we are using an arming switch, for use by set_throttle_zero_flag
+                ap.armed_with_switch = true;
                 break;
             case AUX_SWITCH_LOW:
                 init_disarm_motors();