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/mx6ulz_14x14_evk_defconfig b/configs/mx6ulz_14x14_evk_defconfig
index 2bbcaa6..e06d55f 100644
--- a/configs/mx6ulz_14x14_evk_defconfig
+++ b/configs/mx6ulz_14x14_evk_defconfig
@@ -26,6 +26,7 @@
 CONFIG_CMD_FAT=y
 CONFIG_CMD_FS_GENERIC=y
 CONFIG_OF_CONTROL=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