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/MPC8315ERDB_defconfig b/configs/MPC8315ERDB_defconfig
index d7981e9..f03fec9 100644
--- a/configs/MPC8315ERDB_defconfig
+++ b/configs/MPC8315ERDB_defconfig
@@ -129,6 +129,7 @@
 CONFIG_CMD_MTDPARTS=y
 CONFIG_MTDIDS_DEFAULT="nand0=e0600000.flash"
 CONFIG_MTDPARTS_DEFAULT="mtdparts=e0600000.flash:512k(uboot),128k(env),6m@1m(kernel),-(fs)"
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_ADDR=0xFE080000
 CONFIG_FSL_SATA=y
 # CONFIG_MMC is not set