Improve the error message when Xilinx ISE is not found

[mithro]

Xilinx ISE is currently expected to be found in it's default install path of /op/Xilinx/14.7/ISE_DS. If ISE is not found in this location, the error message should be more informative and explain how to fix the problem. (Either install ISE to the default location, or set XILINX_DIR parameter?)

[shivmgg]

Hi, I think I can solve this issue. The following message can be added in the download-env.sh script: "Either reinstall ISE in the default location or soft link the current installation directory by following the steps mentioned below:
$ mkdir /opt/Xilinx
$ mkdir /opt/Xilinx/14.7
$ ln -s $current_installation_dir_ISE /opt/Xilinx"

We can first check the existence of ISE in the default location and then warn them to follow these steps for correct installation.
Similarly, we can add the option for Xilinx Vivado. Should I commit these changes?