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/10m50_defconfig b/configs/10m50_defconfig
index e7ac7fa..fd84dcf 100644
--- a/configs/10m50_defconfig
+++ b/configs/10m50_defconfig
@@ -20,6 +20,7 @@
 # CONFIG_CMD_NFS is not set
 CONFIG_CMD_MII=y
 CONFIG_CMD_PING=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_ENV_ADDR=0xF4080000
 CONFIG_NET_RANDOM_ETHADDR=y