mntreform-config: set CONFIG_ENV_IS_NOWHERE=y

This is to avoid that u-boot reads the environment from emmc. This is another step to make sure that accidentally running "saveenv" doesn't mess anything up. It also makes sure that a u-boot image flashed to the SD-Card really provides a pristine environment without external influence.

This still needs to be tested by somebody who messed up their environment on emmc by using saveenv in error. I'm too much a coward to try this out myself on my own unit.