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/clearfog_defconfig b/configs/clearfog_defconfig
index 643b7db..fd8eff6 100644
--- a/configs/clearfog_defconfig
+++ b/configs/clearfog_defconfig
@@ -41,6 +41,7 @@
 CONFIG_CMD_TIME=y
 CONFIG_CMD_MVEBU_BUBT=y
 # CONFIG_SPL_PARTITION_UUIDS is not set
+CONFIG_ENV_OVERWRITE=y
 CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_SPL_OF_TRANSLATE=y
 CONFIG_AHCI_MVEBU=y