Copter: ignore first ekf core switch

The ekf core is initialised to -1 but after initialisation changes to zero.  Ignore this first change.

ArduCopter/Copter.h

@@ -223,7 +223,7 @@ private:
 
     // system time in milliseconds of last recorded yaw reset from ekf
     uint32_t ekfYawReset_ms = 0;
-    uint8_t ekf_primary_core;
+    int8_t ekf_primary_core;
 
     // GCS selection
     AP_SerialManager serial_manager;

ArduCopter/ekf_check.cpp

@@ -183,10 +183,10 @@ void Copter::check_ekf_reset()
 
 #if AP_AHRS_NAVEKF_AVAILABLE
     // check for change in primary EKF (log only, AC_WPNav handles position target adjustment)
-    if (EKF2.getPrimaryCoreIndex() != ekf_primary_core) {
+    if ((EKF2.getPrimaryCoreIndex() != ekf_primary_core) && (EKF2.getPrimaryCoreIndex() != -1)) {
         ekf_primary_core = EKF2.getPrimaryCoreIndex();
         Log_Write_Error(ERROR_SUBSYSTEM_EKF_PRIMARY, ekf_primary_core);
-        gcs_send_text_fmt(MAV_SEVERITY_WARNING, "EKF primary changed:%u\n", (unsigned)ekf_primary_core);
+        gcs_send_text_fmt(MAV_SEVERITY_WARNING, "EKF primary changed:%d\n", (unsigned)ekf_primary_core);
     }
 #endif
 }