Plane: fixed issue where auto is entered before GPS lock

thanks to Tom Pittenger for noticing this!

ArduPlane/ArduPlane.pde

@@ -530,8 +530,9 @@ static struct {
     // denotes if a go-around has been commanded for landing
     bool commanded_go_around:1;
 
-    // Altitude threshold to complete a takeoff command in autonomous modes.  Centimeters
-    int32_t takeoff_altitude_cm;
+    // Altitude threshold to complete a takeoff command in autonomous
+    // modes.  Centimeters above home
+    int32_t takeoff_altitude_rel_cm;
 
     // Minimum pitch to hold during takeoff command execution.  Hundredths of a degree
     int16_t takeoff_pitch_cd;
@@ -566,7 +567,7 @@ static struct {
     fbwa_tdrag_takeoff_mode : false,
     checked_for_autoland : false,
     commanded_go_around : false,
-    takeoff_altitude_cm : 0,
+    takeoff_altitude_rel_cm : 0,
     takeoff_pitch_cd : 0,
     highest_airspeed : 0,
     initial_pitch_cd : 0,

ArduPlane/altitude.pde

@@ -414,6 +414,16 @@ static int32_t adjusted_altitude_cm(void)
     return current_loc.alt - (g.alt_offset*100);
 }
 
+/*
+  return home-relative altitude adjusted for ALT_OFFSET This is useful
+  during long flights to account for barometer changes from the GCS,
+  or to adjust the flying height of a long mission
+ */
+static int32_t adjusted_relative_altitude_cm(void)
+{
+    return adjusted_altitude_cm() - home.alt;
+}
+
 /*
   return the height in meters above the next_WP_loc altitude
  */

ArduPlane/commands_logic.pde

@@ -300,7 +300,7 @@ static void do_takeoff(const AP_Mission::Mission_Command& cmd)
     set_next_WP(cmd.content.location);
     // pitch in deg, airspeed  m/s, throttle %, track WP 1 or 0
     auto_state.takeoff_pitch_cd        = (int16_t)cmd.p1 * 100;
-    auto_state.takeoff_altitude_cm     = next_WP_loc.alt;
+    auto_state.takeoff_altitude_rel_cm = next_WP_loc.alt - home.alt;
     next_WP_loc.lat = home.lat + 10;
     next_WP_loc.lng = home.lng + 10;
     auto_state.takeoff_speed_time_ms = 0;
@@ -398,7 +398,10 @@ static bool verify_takeoff()
     }
 
     // see if we have reached takeoff altitude
-    if (adjusted_altitude_cm() > auto_state.takeoff_altitude_cm) {
+    int32_t relative_alt_cm = adjusted_relative_altitude_cm();
+    if (relative_alt_cm > auto_state.takeoff_altitude_rel_cm) {
+        gcs_send_text_fmt(PSTR("Takeoff complete at %.2fm"), 
+                          relative_alt_cm*0.01f);
         steer_state.hold_course_cd = -1;
         auto_state.takeoff_complete = true;
         next_WP_loc = prev_WP_loc = current_loc;