Convert CONFIG_ENV_OVERWRITE to Kconfig

This converts the following to Kconfig:
   CONFIG_ENV_OVERWRITE

Signed-off-by: Adam Ford <aford173@gmail.com>
[trini: Rerun migration, remove some comments]
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/cm_fx6_defconfig b/configs/cm_fx6_defconfig
index c9c16f9..bdc094c 100644
--- a/configs/cm_fx6_defconfig
+++ b/configs/cm_fx6_defconfig
@@ -54,6 +54,7 @@
 CONFIG_MTDIDS_DEFAULT="nor0=spi0.0"
 CONFIG_MTDPARTS_DEFAULT="mtdparts=spi0.0:768k(uboot),256k(uboot-environment),-(reserved)"
 CONFIG_OF_CONTROL=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG=y