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/MPC8313ERDB_NAND_33_defconfig b/configs/MPC8313ERDB_NAND_33_defconfig
index a314db9..55fe1fc 100644
--- a/configs/MPC8313ERDB_NAND_33_defconfig
+++ b/configs/MPC8313ERDB_NAND_33_defconfig
@@ -151,6 +151,7 @@
 CONFIG_CMD_MTDPARTS=y
 CONFIG_MTDIDS_DEFAULT="nand0=e2800000.flash"
 CONFIG_MTDPARTS_DEFAULT="mtdparts=e2800000.flash:512k(uboot),128k(env),6m@1m(kernel),-(fs)"
+CONFIG_ENV_OVERWRITE=y
 # CONFIG_ENV_IS_IN_FLASH is not set
 CONFIG_ENV_IS_IN_NAND=y
 CONFIG_SYS_REDUNDAND_ENVIRONMENT=y