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/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]));
+}