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/mx6sabresd_defconfig b/configs/mx6sabresd_defconfig
index 6c6b4ac..fe34045 100644
--- a/configs/mx6sabresd_defconfig
+++ b/configs/mx6sabresd_defconfig
@@ -64,6 +64,7 @@
 CONFIG_SPL_MULTI_DTB_FIT=y
 CONFIG_SPL_OF_LIST="imx6dl-sabresd imx6q-sabresd imx6qp-sabresd"
 CONFIG_SPL_MULTI_DTB_FIT_NO_COMPRESSION=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_MMC=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG=y