RC_Channel: improved efficiency of min_max_configured

this is quite a costly function due to scan in AP_Param. It takes
about 4 to 5 milliseconds when done as part of AP_Arming

libraries/RC_Channel/RC_Channel.cpp

@@ -27,6 +27,8 @@ extern const AP_HAL::HAL& hal;
 
 #include "RC_Channel.h"
 
+uint32_t RC_Channel::configured_mask;
+
 const AP_Param::GroupInfo RC_Channel::var_info[] = {
     // @Param: MIN
     // @DisplayName: RC min PWM
@@ -335,3 +337,17 @@ bool RC_Channel::in_trim_dz()
     return is_bounded_int32(radio_in, radio_trim - dead_zone, radio_trim + dead_zone);
 }
 
+
+bool RC_Channel::min_max_configured() const
+{
+    if (configured_mask & (1U << ch_in)) {
+        return true;
+    }
+    if (radio_min.configured() && radio_max.configured()) {
+        // once a channel is known to be configured it has to stay
+        // configured due to the nature of AP_Param
+        configured_mask |= (1U<<ch_in);
+        return true;
+    }
+    return false;
+}

libraries/RC_Channel/RC_Channel.h

@@ -97,10 +97,7 @@ public:
 
     void       set_and_save_trim() { radio_trim.set_and_save_ifchanged(radio_in);}
     
-    bool min_max_configured() const
-    {
-        return radio_min.configured() && radio_max.configured();
-    }
+    bool min_max_configured() const;
     
 private:
 
@@ -123,6 +120,9 @@ private:
     // the input channel this corresponds to
     uint8_t     ch_in;
 
+    // bits set when channel has been identified as configured
+    static uint32_t configured_mask;
+
     int16_t pwm_to_angle();
     int16_t pwm_to_angle_dz(uint16_t dead_zone);
 };