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/dfi-bt700-q7x-151_defconfig b/configs/dfi-bt700-q7x-151_defconfig
index 3b2ec53..11f7b88 100644
--- a/configs/dfi-bt700-q7x-151_defconfig
+++ b/configs/dfi-bt700-q7x-151_defconfig
@@ -46,6 +46,7 @@
 CONFIG_MAC_PARTITION=y
 CONFIG_ISO_PARTITION=y
 CONFIG_EFI_PARTITION=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_REGMAP=y
 CONFIG_SYSCON=y