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/P2020RDB-PC_36BIT_defconfig b/configs/P2020RDB-PC_36BIT_defconfig
index 3ec64f4..e5573cd 100644
--- a/configs/P2020RDB-PC_36BIT_defconfig
+++ b/configs/P2020RDB-PC_36BIT_defconfig
@@ -34,6 +34,7 @@
 CONFIG_MTDIDS_DEFAULT="nor0=fef000000.nor"
 CONFIG_MTDPARTS_DEFAULT="mtdparts=fef000000.nor:256k(vsc7385-firmware),256k(dtb),4608k(kernel),9984k(fs),1280k(u-boot)"
 CONFIG_OF_CONTROL=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_ENV_ADDR=0xEFF20000
 CONFIG_DM=y