AP_HAL_ChibiOS: allow AP_TERRAIN_ENABLED to be turned off in hwdef

adds the ifndef, and changes things to the path defines are undefined if it is false

libraries/AP_HAL_ChibiOS/hwdef/scripts/defaults_normal.h

@@ -10,9 +10,18 @@
 #define HAL_BOARD_LOG_DIRECTORY "/APM/LOGS"
 #endif
 
-#ifndef HAL_BOARD_TERRAIN_DIRECTORY
-#define HAL_BOARD_TERRAIN_DIRECTORY "/APM/TERRAIN"
+// a similar define is present in AP_HAL_Boards.h:
+#ifndef HAL_OS_FATFS_IO
+#define HAL_OS_FATFS_IO 0
 #endif
 
+#ifndef AP_TERRAIN_AVAILABLE
 // enable terrain only if there's an SD card available:
 #define AP_TERRAIN_AVAILABLE HAL_OS_FATFS_IO
+#endif
+
+#if AP_TERRAIN_AVAILABLE
+#ifndef HAL_BOARD_TERRAIN_DIRECTORY
+#define HAL_BOARD_TERRAIN_DIRECTORY "/APM/TERRAIN"
+#endif
+#endif  // AP_TERRAIN_AVAILABLE