Support .XCI files for modified IPs. #1165

Hi,

There is a capability in the Vivado which users can modify IP sources and lock the IP to not be reset by other users or tools.

For example in some cases it is required to remove local clock buffers inside the IPs to be able to share the FPGA clock pin between two IPs and implement clock buffers on the top.

The way of modifying and locking the IP is clarified in the documentation and .xci should be used in this case as a source. When we are creating Tcl files, all the sources will be generated again and it is not possible to use modified IPs.

I cannot find another solution for adding these kind of modified IPs to the building and simulation flow of Vunit and it looks that .xci file should be supported in Vunit.