littlefs: Adopted the block device sync function

Required to garuntee that data is flushed all the way down to the disk
level when a file is synced or closed.

Author: geky

features/filesystem/littlefs/LittleFileSystem.cpp

@@ -102,7 +102,8 @@ static int lfs_bd_erase(const struct lfs_config *c, lfs_block_t block)
 
 static int lfs_bd_sync(const struct lfs_config *c)
 {
-    return 0;
+    BlockDevice *bd = (BlockDevice *)c->context;
+    return bd->sync();
 }