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/brppt1_nand_defconfig b/configs/brppt1_nand_defconfig
index ba64212..aa4a1b2 100644
--- a/configs/brppt1_nand_defconfig
+++ b/configs/brppt1_nand_defconfig
@@ -67,6 +67,7 @@
 CONFIG_OF_CONTROL=y
 CONFIG_SPL_OF_CONTROL=y
 CONFIG_OF_SPL_REMOVE_PROPS="pinctrl-0 pinctrl-names clocks clock-names interrupt-parent interrupt-controller interrupt-cells dma-names dmas "
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_NAND=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_NET_RANDOM_ETHADDR=y