AP_Compass: disable auto-declination on 1280

this saves a few k of flash space

libraries/AP_Compass/Compass.cpp

@@ -8,7 +8,9 @@ const AP_Param::GroupInfo Compass::var_info[] PROGMEM = {
     AP_GROUPINFO("DEC",    2, Compass, _declination, 0),
     AP_GROUPINFO("LEARN",  3, Compass, _learn, 1), // true if learning calibration
     AP_GROUPINFO("USE",    4, Compass, _use_for_yaw, 1), // true if used for DCM yaw
+#if !defined( __AVR_ATmega1280__ )
     AP_GROUPINFO("AUTODEC",5, Compass, _auto_declination, 1),
+#endif
     AP_GROUPEND
 };
 
@@ -61,6 +63,7 @@ Compass::set_initial_location(int32_t latitude, int32_t longitude)
 {
     // if automatic declination is configured, then compute
     // the declination based on the initial GPS fix
+#if !defined( __AVR_ATmega1280__ )
     if (_auto_declination) {
         // Set the declination based on the lat/lng from GPS
         _declination.set(radians(
@@ -68,6 +71,7 @@ Compass::set_initial_location(int32_t latitude, int32_t longitude)
                     (float)latitude / 10000000,
                     (float)longitude / 10000000)));
     }
+#endif
 }
 
 void