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/colibri_imx7_defconfig b/configs/colibri_imx7_defconfig
index 5fe14a7..b2cce8a 100644
--- a/configs/colibri_imx7_defconfig
+++ b/configs/colibri_imx7_defconfig
@@ -49,6 +49,7 @@
 CONFIG_CMD_UBI=y
 # CONFIG_ISO_PARTITION is not set
 CONFIG_OF_CONTROL=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_NAND=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG=y