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/mt7622_rfb_defconfig b/configs/mt7622_rfb_defconfig
index 249daab92..9ab621d 100644
--- a/configs/mt7622_rfb_defconfig
+++ b/configs/mt7622_rfb_defconfig
@@ -16,6 +16,7 @@
 CONFIG_CMD_SF_TEST=y
 CONFIG_CMD_PING=y
 CONFIG_CMD_SMC=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG=y
 CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_REGMAP=y