AP_HAL: added AP_HAL_BOARD_DRIVER define

this makes example sketches easier to read

libraries/AP_HAL/AP_HAL_Boards.h

@@ -15,5 +15,21 @@
 #define HAL_BOARD_AVR_SITL 3
 #define HAL_BOARD_EMPTY    99
 
+/*
+  define AP_HAL_BOARD_DRIVER to the right hal type for this
+  board. This prevents us having a mess of ifdefs in every example
+  sketch
+ */
+
+#if CONFIG_HAL_BOARD == HAL_BOARD_APM1
+#define AP_HAL_BOARD_DRIVER AP_HAL_AVR_APM1
+#elif CONFIG_HAL_BOARD == HAL_BOARD_APM2
+#define AP_HAL_BOARD_DRIVER AP_HAL_AVR_APM2
+#elif CONFIG_HAL_BOARD == HAL_BOARD_AVR_SITL
+#define AP_HAL_BOARD_DRIVER AP_HAL_AVR_SITL
+#else
+#error "Unknown CONFIG_HAL_BOARD type"
+#endif
+
 #endif // __AP_HAL_BOARDS_H__