Enable log to stdout/stderr for dataflow builder, to enable PDB debugging

[quetric]

When debugging new FINN code it's helpful to get a debugger console and walk through the code. ATM the dataflow builder prevents this by redirecting debugger output to the log.

I was able to circumvent this by commenting out lines 108/109 and assigning the stdout and stderr loggers to sys.stdout and sys.stderr.

https://github.com/Xilinx/finn/blob/0fc686e9d0ed826046da7ab38f339148ea340afa/src/finn/builder/build_dataflow.py#L108