Plane: added relative_altitude helper functions

prevents some code duplication

ArduPlane/ArduPlane.pde

@@ -785,7 +785,7 @@ static void update_speed_height(void)
     if (g.alt_control_algorithm == ALT_CONTROL_TECS && auto_throttle_mode && !throttle_suppressed) 
     {
 	    // Call TECS 50Hz update
-        SpdHgt_Controller->update_50hz((current_loc.alt - home.alt) * 0.01f);
+        SpdHgt_Controller->update_50hz(relative_altitude());
     }
 }
 
@@ -1255,7 +1255,7 @@ static void update_alt()
                                                  (control_mode==AUTO && takeoff_complete == false), 
                                                  takeoff_pitch_cd,
                                                  throttle_nudge,
-                                                 float(hgt_afe_cm)*0.01f);
+                                                 relative_altitude());
         if (g.log_bitmask & MASK_LOG_TECS) {
             Log_Write_TECS_Tuning();
         }

ArduPlane/Attitude.pde

@@ -257,7 +257,7 @@ static void stabilize()
       see if we should zero the attitude controller integrators. 
      */
     if (channel_throttle->control_in == 0 &&
-        abs(current_loc.alt - home.alt) < 500 && 
+        relative_altitude_abs_cm() < 500 && 
         fabs(barometer.get_climb_rate()) < 0.5f &&
         g_gps->ground_speed < 300) {
         // we are low, with no climb rate, and zero throttle, and very
@@ -511,7 +511,7 @@ static bool suppress_throttle(void)
         return false;
     }
     
-    if (labs(home.alt - current_loc.alt) >= 1000) {
+    if (relative_altitude_abs_cm() >= 1000) {
         // we're more than 10m from the home altitude
         throttle_suppressed = false;
         return false;

ArduPlane/GCS_Mavlink.pde

@@ -264,8 +264,8 @@ static void NOINLINE send_location(mavlink_channel_t chan)
         fix_time,
         current_loc.lat,                // in 1E7 degrees
         current_loc.lng,                // in 1E7 degrees
-        g_gps->altitude * 10,             // millimeters above sea level
-        (current_loc.alt-home.alt) * 10,           // millimeters above ground
+        g_gps->altitude * 10,           // millimeters above sea level
+        relative_altitude() * 1.0e3,    // millimeters above ground
         g_gps->velocity_north() * 100,  // X speed cm/s (+ve North)
         g_gps->velocity_east()  * 100,  // Y speed cm/s (+ve East)
         g_gps->velocity_down()  * -100, // Z speed cm/s (+ve up)

ArduPlane/navigation.pde

@@ -193,3 +193,20 @@ static void setup_glide_slope(void)
         break;
     }
 }
+
+/*
+  return relative altitude in meters (relative to home)
+ */
+static float relative_altitude(void)
+{
+    return (current_loc.alt - home.alt) * 0.01f;
+}
+
+/*
+  return relative altitude in centimeters, absolute value
+ */
+static int32_t relative_altitude_abs_cm(void)
+{
+    return labs(current_loc.alt - home.alt);
+}
+