Relax output suppression (--loglevel) to not suppress Vivado simulation results

[jhegeman]

The output formatter used for Vivado output is a bit harsh. Any line that is not actively identified as a specific log level is suppressed unless the log level is set looser than INFO.
One of the side-effects of this choice is that even with --loglevel=INFO, all Vivado simulation output is suppressed. (Unless one gets into the habit of prefixing simulation output with 'INFO'.)

In my opinion, anything not explicitly marked with a log level should be considered informational. So I'd like to propose changing the '>=' in the following line to '>'.
https://github.com/ipbus/ipbb/blob/develop/src/ipbb/tools/xilinx/vivado_common.py#L161