"remove  #ifdef RT_SERIAL_USING_DMA in rt_serial_open， always check device open dma flag"

rx_fifo->is_full已经满时继续更新rx_fifo->get_index

Fix compile error when RT_USING_POSIX_TERMIOS is not define.

1.Fix poll rx issue when data is full.
2.Add TCFLSH and FIONREAD support.

data bits should get from CSIZE.

data bits should get from CSIZE.

1. Add macro check in rtdebug.h;
2. Use RT_USING_POSIX for poll/select, stdin etc.
3. Split dfs_posix.h to dfs_posix.h, dfs_poll.h and dfs_select.h;

1. Change 'rt_uint8_t' type of cmd to 'int';
2. Add waitqueue;
3. Split device ipc header files;

uart configure shall not been performed if uart is not open.

fix reset serial receive interrupt flag when close serial

This reverts commit 5aeedc4f, reversing
changes made to 3d80e414.

This merge breaks every uart driver in all BSPs. @bright-pan , is there
any way to get this done without breaking existing code?

Add hardware flow support.
Use new struct serial_ringbuffer.

This reverts commit 07bd09fc.
The validation of the context is done with in `rt_set_errno`. So it's
safe to call it here.