Add MTD loop driver (use filemtd)

drivers/drivers_initialize.c

@@ -29,6 +29,7 @@
 #include <nuttx/drivers/rpmsgblk.h>
 #include <nuttx/fs/loop.h>
 #include <nuttx/fs/smart.h>
+#include <nuttx/fs/loopmtd.h>
 #include <nuttx/input/uinput.h>
 #include <nuttx/mtd/mtd.h>
 #include <nuttx/net/loopback.h>
@@ -189,4 +190,8 @@ void drivers_initialize(void)
 #ifdef CONFIG_SMART_DEV_LOOP
   smart_loop_register_driver();
 #endif
+
+#ifdef CONFIG_MTD_LOOP
+  mtd_loop_register();
+#endif
 }