Xilinx libraries need work about kicad-symbols

[bobc]

I've nearly got symgen to the point where it can be easily used to clean up these type of libraries, certainly it can benefit from exposure to more test cases. symgen fixes most of the KLC style errors, mainly leaving missing doc fields and footprints.

Unfortunately I have a backlog of other libraries to process, if it would be useful I can document the process for others.

Note that Xilinx provide text files with pin definitions (no other data), which can be read by KiPart to produce libraries but they are not very KLC compliant. I have adapted KiPart to produce symgen scripts as well, which might be more useful creating KLC libs.