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/ae350_rv32_spl_defconfig b/configs/ae350_rv32_spl_defconfig
index bb9be78..ff8d407 100644
--- a/configs/ae350_rv32_spl_defconfig
+++ b/configs/ae350_rv32_spl_defconfig
@@ -17,6 +17,7 @@
 CONFIG_BOOTP_PREFER_SERVERIP=y
 CONFIG_CMD_CACHE=y
 CONFIG_OF_PRIOR_STAGE=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_MMC=y