ringbuffer: fix the ambiguous name

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.
ringbuffer: add put_force and putchar_force API

Add the APIs that will discard the old data when rb is full.
device/pipe: add nonblocking read/write and force write mode

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.
dev/pipe: call rx_indicate the time we resume reader

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.
dev/pipe: cleanup
Merge pull request #143 from grissiom/ringbuffer-pipe

Ringbuffer and Pipe