fpga: Multiple X300 FPGA bugfixes and enhancements

- Fixed 10GigE firmware communication issues and sequence errors for TX
- Multiple changes to help ease timing closure
- Cleaned up build scripts
- Switched to Xilinx ISE 14.7 as the default build tool for X300
docs: Changed Xilinx ISE version from 14.4 to 14.7
Updated ChangeLog for 3.7.3
Updated images URL for 3.7.3RC1