Drop Xilinx ISE Support

[Paebbels]

Xilinx ISE (last version 14.7 from September 2013) is more than outdated.
PoC should drop Xilinx ISE support, specific IP-Cores and undo ISE specific workarounds.

• Drop Xilinx ISE from documentation and scripting.
• Undo Xilinx ISE workarounds.
• Drop Xilinx specific IP cores (MIG, ...)