AP_AHRS: try to start EKF2 slightly before EKF2

this gives priority to EKF2 on memory

libraries/AP_AHRS/AP_AHRS.h

@@ -412,6 +412,11 @@ public:
     // time that the AHRS has been up
     virtual uint32_t uptime_ms(void) const = 0;
 
+    // get the selected ekf type, for allocation decisions
+    int8_t get_ekf_type(void) const {
+        return _ekf_type;
+    }
+    
 protected:
     AHRS_VehicleClass _vehicle_class;
 

libraries/AP_AHRS/AP_AHRS_NavEKF.cpp

@@ -109,7 +109,8 @@ void AP_AHRS_NavEKF::update_EKF1(void)
         if (start_time_ms == 0) {
             start_time_ms = AP_HAL::millis();
         }
-        if (AP_HAL::millis() - start_time_ms > startup_delay_ms) {
+        // slight extra delay on EKF1 to prioritise EKF2 for memory
+        if (AP_HAL::millis() - start_time_ms > startup_delay_ms + 100) {
             ekf1_started = EKF1.InitialiseFilterDynamic();
         }
     }