config: add "fat_lfn" feature

libconfig/Cargo.toml

@@ -16,3 +16,4 @@ target_coraz7 = []
 target_redpitaya = []
 target_kasli_soc = []
 ipv6 = []
+fat_lfn = [ "fatfs/alloc" ]

libconfig/src/lib.rs

@@ -164,7 +164,8 @@ impl Config {
                 f.seek(SeekFrom::End(0))?;
                 write!(f, "{}={}\n", key, String::from_utf8(value).unwrap())?;
             } else {
-                let mut f = root_dir.create_file(&["/CONFIG/", key, ".BIN"].concat())?;
+                let dir = root_dir.create_dir("/CONFIG")?;
+                let mut f = dir.create_file(&[key, ".BIN"].concat())?;
                 f.write_all(&value)?;
             }
         }