device/pipe: add a control cmd to get the space left in pipe
log_trace: add a in-mem-log example

After `log_trace_init()`, call `memlog_init`. It then turn the logtrace
into a "in-memory" logger which will buffer all the log in memory. It
also set a hook in idle to flush all the log into console. One may
create an other thread to flush the logs but idle might be the simplest
place to go.
Merge pull request #183 from grissiom/log_trace-in-mem

Log trace in mem