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/MPC8568MDS_defconfig b/configs/MPC8568MDS_defconfig
index 820bd72..e41ba36 100644
--- a/configs/MPC8568MDS_defconfig
+++ b/configs/MPC8568MDS_defconfig
@@ -19,6 +19,7 @@
 CONFIG_CMD_MII=y
 CONFIG_CMD_PING=y
 # CONFIG_CMD_HASH is not set
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_ENV_ADDR=0xFFF60000
 # CONFIG_MMC is not set