CONTRIB: trace: add the possibility to place trace calls in the code

CONTRIB: trace: add the possibility to place trace calls in the code

Now any call to trace() in the code will automatically appear interleaved
with the call sequence and timestamped in the trace file. They appear with
a '#' on the 3rd argument (caller's pointer) in order to make them easy to
spot. If the trace functionality is not used, a dmumy weak function is used
instead so that it doesn't require to recompile every time traces are
enabled/disabled.

The trace decoder knows how to deal with these messages, detects them and
indents them similarly to the currently traced function. This can be used
to print function arguments for example.

Note that we systematically flush the log when calling trace() to ensure we
never miss important events, so this may impact performance.

The trace() function uses the same format as printf() so it should be easy
to setup during debugging sessions.