Fix bug for VFS & FatFS component

[KY-zhang-X]

• 目前的 .gitignore 不能忽略 board/Keil 目录下的文件，commit 9f24c1f 和 24d7a9c 中做了一些修改
• components/fs/vfs/tos_vfs.c 中的 tos_vfs_readdir 函数会直接退出，commit 3c1eac5 中做了修改
• components/fs/fatfs/wrapper/tos_fatfs_vfs.c 中的 fatfs_readdir 函数不能在目录读取结束后停止，原因是 tos_f_readdir 函数的返回值不能代表目录是否读取完毕，commit 3c1eac5 中做了修改
• components/fs/fatfs/wrapper/tos_ff.c 中的 tos_f_stat 函数无法返回正确结果，偶尔触发 stm32 的 HardFault 异常，原因是没有给 dj.obj.fs 变量赋值，commit 30988eb 中做了修改

[tencent-adm]

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[Supowang1989]

Thanks for your contribution