Simulator linux

SDL may use DFS to open files. It should be run after
rt_components_init.

rt_hw_sdl_start should wait untill the rtgui_graphic_set_device is
called int the sdl_loop. Use pthread_cond_wait to achieve this in Linux.

Some fix for the portal device

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.

finsh/cmd: add description for new pipe and portal device

Portal device

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.

Ringbuffer and Pipe

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.

device/pipe: add rt_pipe_{init,detach}

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.

fix debug output of scheduler

When print the thread names, we should use "%.*s" instead of bare "%s".

SEP6200 Support

K60 fxxxx