Disable I2C for the next boot

I need I2C to be normally enabled (in the device tree?) so that I2C driver loads for it, but at some cases I need those pins to be GPIOs (to bitbang them). I found this but fw_printenv says missing /etc/fw_env.conig file (where does it originate?) and this saying that I can't disable/unload I2C during run time. I am ok to have modified behavior after the reboot.

What are the best and/or most universal/portable options for this?