change rc norm_output to scale evenly across the entire range

ArduPlane/GCS_Mavlink.pde

@@ -420,7 +420,7 @@ static void NOINLINE send_vfr_hud(mavlink_channel_t chan)
         (float)airspeed / 100.0,
         (float)g_gps->ground_speed / 100.0,
         (dcm.yaw_sensor / 100) % 360,
-        (uint16_t)(100 * g.channel_throttle.norm_output()),
+        (uint16_t)(100 * (g.channel_throttle.norm_output() / 2.0 + 0.5)), // scale -1,1 to 0-100
         current_loc.alt / 100.0,
         0);
 }

libraries/RC_Channel/RC_Channel.cpp

@@ -248,10 +248,12 @@ RC_Channel::norm_input()
 float
 RC_Channel::norm_output()
 {
+    uint16_t mid = (radio_max + radio_min) / 2;
+    
 	if(radio_out < radio_trim)
-		return (float)(radio_out - radio_trim) / (float)(radio_trim - radio_min);
+		return (float)(radio_out - mid) / (float)(mid - radio_min);
 	else
-		return (float)(radio_out - radio_trim) / (float)(radio_max  - radio_trim);
+		return (float)(radio_out - mid) / (float)(radio_max  - mid);
 }
 
 void RC_Channel::set_apm_rc( APM_RC_Class * apm_rc )