ESP32 support: setting env var ESP_DEVICE not working

[xixidodo]

refer to doc
http://docs.zephyrproject.org/boards/xtensa/esp32/doc/esp32.html

Quoted as:
"Environment variables can be set to set the serial port device, baud rate, and other options. Please refer to the following table for details."

after setting ESP_DEVICE=/dev/ttyUSB1, 'ninja flash' still flashing to port ttyUSB0.

Since I can not find ESP_DEVICE env setting in code, I am wondering if the doc is out-of-date or I used it wrongly.

thanks.