GTP support for Artix7, BUFHCE support, fixes

Details see:
https://github.com/openXC7/nextpnr-xilinx/releases/tag/0.8.0

Release 0.7.0

• Upgrade Yosys 0.17 to Yosys 0.36, since its issues with Xilinx Series 7 FPGAs have been resolved
• MMCME2_ADV support
• OSERDESE2 fixes (OFB support)
• Better error messages

Release 0.5.1

See release notes of nexpnr-xilinx

Release 0.5.0

bump to latest release 0.5.0

Release 0.4.0

update uninstaller to openxc7