Is it possible to set the CONFIG STEPPING constraint in an environment variable?
The XIL_MAP_SETSTEPPING environment variable can be used to set the configuration stepping level at a value of "ES", "0", or "1". This environment variable has a higher priority than the CONFIG STEPPING constraint set in the UCF.
For more information on the CONFIG STEPPING constraint, see (Xilinx Answer 14339).
For general information about setting ISE environment variables, see (Xilinx Answer 11630).