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.

mntreform-config

@@ -654,11 +654,12 @@ CONFIG_MKIMAGE_DTC_PATH="dtc"
 #
 # Environment
 #
+CONFIG_ENV_IS_NOWHERE=y
 # CONFIG_ENV_IS_IN_EEPROM is not set
 # CONFIG_ENV_IS_IN_FAT is not set
 # CONFIG_ENV_IS_IN_EXT4 is not set
 # CONFIG_ENV_IS_IN_FLASH is not set
-CONFIG_ENV_IS_IN_MMC=y
+# CONFIG_ENV_IS_IN_MMC is not set
 # CONFIG_ENV_IS_IN_NAND is not set
 # CONFIG_ENV_IS_IN_NVRAM is not set
 # CONFIG_ENV_IS_IN_ONENAND is not set