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/MPC837XERDB_SLAVE_defconfig b/configs/MPC837XERDB_SLAVE_defconfig
index 87fe4fc..ea84564 100644
--- a/configs/MPC837XERDB_SLAVE_defconfig
+++ b/configs/MPC837XERDB_SLAVE_defconfig
@@ -119,6 +119,7 @@
 CONFIG_CMD_DATE=y
 CONFIG_CMD_EXT2=y
 CONFIG_CMD_FAT=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_ADDR=0xFE080000
 CONFIG_FSL_SATA=y
 CONFIG_MTD_NOR_FLASH=y