OXape reports: http://www.rt-thread.org/phpBB3/topic3939.html
          driver/ringbuffer: when data length is bigger than space length
          for rb, the read_index in rb is not update.
    When length > space, update the read_index, the bug can be fix, and
          works fine.

2. Fix data transfer size calculate error issue;

2.adjust some viable and micro name
3.some other small fix

This reverts commit 5aeedc4f3a3c3fcfffb0cd4a474f5faa75053839, reversing
changes made to 3d80e414ff6c03ff04fecb914e7aa9b3fb535e03.

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.

Pass USB CV test verification;
Code cleanup;

This reverts commit 07bd09fc0fc256e5c53ad3c6157802fd94c9910c.
The validation of the context is done with in `rt_set_errno`. So it's
safe to call it here.

The errno is associated with threads. It will crash the thread if we set
it in interrupt context.

Return early if the size is 0 as there is nothing to do.

*buffer) with buffer pointer

When forwarding rx/tx callback from the underlaying device(pipe), the
"dev" argument should be the portal. So the portal callback could think
itself as called.

It does not make sense to open portal without a flag. One should open a
portal with RT_DEVICE_OFLAG_RDWR in most cases.

Portal is a device that connect devices. Currently, you can only connect
pipes in portal. Pipes are unidirectional. But with portal, you can
construct a bidirectional device with two pipes.

Pipe have many differences with char device. The main difference is
altough pipe have both read and write, it is unidirectional in nature.

Some reader(FinSh again) would like to block on a semaphore which is
released in rx_indicate. So we should invoke rx_indicate in
_rt_pipe_resume_reader.

The previous implementation will always blocks the reader/writer.
However, at least FinSh would expect the device to be nonblocking ---
read should return 0 when there is no data in it.

Add the APIs that will discard the old data when rb is full.

RT_RINGBUFFER_SIZE could mean "the size of the whole buffer", "the size
of the empty space" or "the size of the data". Moreover, it's never a
micro anymore. Change it to rt_ringbuffer_data_len before it's too late.
Also, RT_RINGBUFFER_EMPTY is changed to rt_ringbuffer_space_len.

This provide the possibility that allocate the buffer of the ringbuffer
on a specific region, instead of always mallocing it. It also bring us
the benefit of using pipe device on the systems without heap.