doc: fix doc hierarchy

drivers/include/mtd.h

@@ -7,7 +7,8 @@
  */
 
 /**
- * @defgroup    mtd Memory Technology Device
+ * @defgroup    drivers_mtd Memory Technology Device
+ * @ingroup     drivers_storage
  * @{
  * @brief       Low level Memory Technology Device interface
  *

sys/fs/constfs/constfs.c

@@ -7,7 +7,7 @@
  */
 
 /**
- * @ingroup     fs_constfs
+ * @ingroup     sys_fs_constfs
  * @{
  *
  * @file

sys/fs/devfs/devfs.c

@@ -7,7 +7,7 @@
  */
 
 /**
- * @ingroup     fs_devfs
+ * @ingroup     sys_fs_devfs
  * @{
  *
  * @file

sys/fs/doc.txt

@@ -7,6 +7,7 @@
  */
 
 /**
- * @defgroup    fs File systems
+ * @defgroup    sys_fs File systems
+ * @ingroup     sys
  * @brief       File system libraries
  */

sys/include/fs/constfs.h

@@ -7,8 +7,8 @@
  */
 
 /**
- * @defgroup  fs_constfs ConstFS static file system
- * @ingroup   fs
+ * @defgroup  sys_fs_constfs ConstFS static file system
+ * @ingroup   sys_fs
  * @brief     Constant file system resident in arrays
  *
  * This is an example of how to implement a simple file system driver for the

sys/include/fs/devfs.h

@@ -7,8 +7,8 @@
  */
 
 /**
- * @defgroup  fs_devfs DevFS device file system
- * @ingroup   fs
+ * @defgroup  sys_fs_devfs DevFS device file system
+ * @ingroup   sys_fs
  * @brief     Dynamic device file system
  *
  * This file system implementation allows devices to register file names for