AP_Module: added external module hook library

this allows for external modules to be called at defined hook
locations in ArduPilot. The initial example is a module that consumes
the AHRS state, but this can be generalised to a wide variety of hooks

libraries/AP_Module/examples/ModuleTest/Makefile

@@ -0,0 +1 @@
+include ../../../../mk/apm.mk

libraries/AP_Module/examples/ModuleTest/ModuleTest.cpp

@@ -0,0 +1,38 @@
+// -*- tab-width: 4; Mode: C++; c-basic-offset: 4; indent-tabs-mode: nil -*-
+
+//
+// Simple test for the AP_AHRS interface
+//
+
+#include <AP_ADC/AP_ADC.h>
+#include <AP_AHRS/AP_AHRS.h>
+#include <AP_HAL/AP_HAL.h>
+#include <AP_Module/AP_Module.h>
+
+const AP_HAL::HAL& hal = AP_HAL::get_HAL();
+
+// sensor declaration
+AP_InertialSensor ins;
+AP_GPS gps;
+AP_Baro baro;
+AP_SerialManager serial_manager;
+
+// choose which AHRS system to use
+AP_AHRS_DCM  ahrs(ins, baro, gps);
+
+void setup(void)
+{
+    serial_manager.init();
+    ins.init(100);
+    baro.init();
+    ahrs.init();
+
+    gps.init(NULL, serial_manager);
+}
+
+void loop(void)
+{
+    ahrs.update();
+}
+
+AP_HAL_MAIN();

libraries/AP_Module/examples/ModuleTest/module/Makefile

@@ -0,0 +1,13 @@
+
+# minimal makefile setup for ARM linux targets
+
+CC=arm-linux-gnueabihf-gcc
+CFLAGS=-Wall -fPIC -g -shared
+
+all: moduletest.so
+
+moduletest.so: moduletest.c
+	$(CC) $(CFLAGS) -o moduletest.so moduletest.c -ldl
+
+clean:
+	rm -f moduletest.so

libraries/AP_Module/examples/ModuleTest/module/moduletest.c

@@ -0,0 +1,37 @@
+/*
+  very simple example module
+ */
+
+#include <stdio.h>
+#include <stdint.h>
+#include <stdbool.h>
+#include <math.h>
+
+#include "../../../AP_Module_Structures.h"
+
+void hook_setup_start(uint64_t time_us)
+{
+    printf("setup_start called\n");
+}
+
+void hook_setup_complete(uint64_t time_us)
+{
+    printf("setup_complete called\n");
+}
+
+
+#define degrees(x) (x * 180.0 / M_PI)
+
+void hook_AHRS_update(const struct AHRS_state *state)
+{
+    static uint64_t last_print_us;
+    if (state->time_us - last_print_us < 1000000UL) {
+        return;
+    }
+    last_print_us = state->time_us;
+    // print euler angles once per second
+    printf("AHRS_update (%.1f,%.1f,%.1f)\n",
+           degrees(state->eulers[0]),
+           degrees(state->eulers[1]),
+           degrees(state->eulers[2]));
+}

libraries/AP_Module/examples/ModuleTest/wscript

@@ -0,0 +1,7 @@
+#!/usr/bin/env python
+# encoding: utf-8
+
+def build(bld):
+    bld.ap_example(
+        use='ap',
+    )