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/d2net_v2_defconfig b/configs/d2net_v2_defconfig
index 284ea96..7823355 100644
--- a/configs/d2net_v2_defconfig
+++ b/configs/d2net_v2_defconfig
@@ -35,6 +35,7 @@
 CONFIG_EFI_PARTITION=y
 # CONFIG_PARTITION_UUIDS is not set
 CONFIG_OF_CONTROL=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_USE_ENV_SPI_MAX_HZ=y
 CONFIG_ENV_SPI_MAX_HZ=20000000