[stm32][pandora] fix the static conflict error in ROMFS

#### 为什么提交这份PR (why to submit this PR)
原版编译报错：Static declaration of 'romfs_root' follows non-static declaration in dfs_romfs.h

原因为：在`board/ports/drv_filesystem.c`71行用`static`定义了常量`romfs_root`，而在`rt-thread/components/dfs/dfs_v1/filesystems/romfs/dfs_romfs.h`29行，有它的外部引用：

```
extern const struct romfs_dirent romfs_root;
```

Original compilation error: Static declaration of 'romfs_ root' follows non-static declaration in dfs_romfs. h

The reason is: in 'board/ports/drv_filesystem.c' line 71 use `static` define `romfs_root`, while in `rt-thread/components/dfs/dfs_v1/filesystems/romfs/dfs_romfs.h` line 29, with its external reference:
```
extern const struct romfs_dirent romfs_root;
```

#### 你的解决方案是什么 (what is your solution)
在`drv_filesystem.c`中，不可以用static修饰这个常量，故去掉`static`关键字。

In  `board/ports/drv_filesystem.c`, it is not allowed to define this constant with static, so the `static` keyword must be removed.

#### 在什么测试环境下测试通过 (what is the test environment)
正点原子潘多拉开发板
stm32l475-atk-pandora

bsp/stm32/stm32l475-atk-pandora/board/ports/drv_filesystem.c

@@ -68,7 +68,7 @@ static const struct romfs_dirent _romfs_root[] =
 #endif
 };
 
-static const struct romfs_dirent romfs_root =
+const struct romfs_dirent romfs_root =
 {
     ROMFS_DIRENT_DIR, "/", (rt_uint8_t *)_romfs_root, sizeof(_romfs_root) / sizeof(_romfs_root[0])
 };