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/MPC832XEMDS_defconfig b/configs/MPC832XEMDS_defconfig
index 53bec93..9554c3f 100644
--- a/configs/MPC832XEMDS_defconfig
+++ b/configs/MPC832XEMDS_defconfig
@@ -127,6 +127,7 @@
 CONFIG_CMD_I2C=y
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_CMD_PING=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_ADDR=0xFE080000
 # CONFIG_MMC is not set
 CONFIG_MTD_NOR_FLASH=y