Add Paparazzi compatibility (#580)

This adds support for the paparazzi autopilot to use this library as EKF.
The header order change has to be done in order to have std.h included to get certain defines at the start of the estimator_interface.h.

EKF/estimator_interface.h

@@ -41,10 +41,10 @@
 
 #pragma once
 
+#include <ecl.h>
 #include "common.h"
 #include "RingBuffer.h"
 
-#include <ecl.h>
 #include <geo/geo.h>
 #include <matrix/math.hpp>
 #include <mathlib/mathlib.h>

ecl.h

@@ -51,6 +51,19 @@ using ecl_abstime = hrt_abstime;
 #define ECL_WARN PX4_WARN
 #define ECL_ERR	 PX4_ERR
 
+#elif defined(__PAPARAZZI)
+
+#include "std.h"
+
+#define ecl_absolute_time() (0)
+#define ecl_elapsed_time(t) (*t * 0UL) // TODO: add simple time functions
+
+using ecl_abstime = uint64_t;
+
+#define ECL_INFO(...)
+#define ECL_WARN(...)
+#define ECL_ERR(...)
+
 #else
 
 #include <cstdio>