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/rcar3_ulcb_defconfig b/configs/rcar3_ulcb_defconfig
index 80a62ef..f9ed9fa 100644
--- a/configs/rcar3_ulcb_defconfig
+++ b/configs/rcar3_ulcb_defconfig
@@ -35,6 +35,7 @@
 CONFIG_OF_LIST="r8a77950-ulcb-u-boot r8a77960-ulcb-u-boot r8a77965-ulcb-u-boot"
 CONFIG_MULTI_DTB_FIT_LZO=y
 CONFIG_MULTI_DTB_FIT_USER_DEFINED_AREA=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_MMC=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_REGMAP=y