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/ls2080ardb_defconfig b/configs/ls2080ardb_defconfig
index 9d40436..77dcf97 100644
--- a/configs/ls2080ardb_defconfig
+++ b/configs/ls2080ardb_defconfig
@@ -31,6 +31,7 @@
 CONFIG_CMD_DATE=y
 CONFIG_MP=y
 CONFIG_OF_CONTROL=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_ENV_ADDR=0x80300000
 CONFIG_NET_RANDOM_ETHADDR=y