1. Add poll/select APIs;
2. Add wait queue implementation;
3. Remove DFS_STATUS_* code and use errno;
4. Add pipe API;
5. Use a standalone fops in DFS;

Fix kconfig

[DFS] To make sure the "console" device exist or not when initialize
console device.

static void copydir(const char * src, const char * dst)

[DFS] update elm fatfs to 0.12b version.

[DFS] Fix the ramfs issue.

修正components.h中使能RT_USING_DFS_RAMFS宏时，加入ramfs.h头文件

比如实际目录为abcde，msh执行 cd abc也能执行成功，判断名称时存在bug

Signed-off-by: pangwei <271372443@qq.com>

To avoid same file name, therefore change the netdb.c to lwip_netdbc in
dfs_lwip.

System will crash when the romfs is erased. Add checks before using them
to avoid it.

Clang complains about "Null pointer argument in call to string
comparison function". So assert on it.

This issue causes fstat & lseek not correct.

I have to mention that this bug is introduced by myself, in the last
commit <fd07f30f511335f506ae6557e300093d4dde421e> to dfs_nfs.c

fix dfs_unmount bug

more infomation:
http://www.rt-thread.org/phpBB3/topic2965.html

fix #211

empty is confused in dfs_filesystem_lookup.

clean code with pointer rather than index

decrease indentation to make cleaner code
fix mkfs bug when there is no mkfs implementation

_TIME_T_DEFINED was pre-defined in order to build jffs2 with VC.
While it may cause many side effects.

found by haitao5198
clear mount parition device talbe when jffs2 mount falied

found by haitao52198
URL:http://www.rt-thread.org/phpBB3/viewtopic.php?f=3&t=3112&p=17153#p17153