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/T2080RDB_SPIFLASH_defconfig b/configs/T2080RDB_SPIFLASH_defconfig
index 1a6941e..62cbab1 100644
--- a/configs/T2080RDB_SPIFLASH_defconfig
+++ b/configs/T2080RDB_SPIFLASH_defconfig
@@ -48,6 +48,7 @@
 CONFIG_MTDPARTS_DEFAULT="mtdparts=fe8000000.nor:1m(uboot),5m(kernel),128k(dtb),96m(fs),-(user);fff800000.flash:1m(uboot),5m(kernel),128k(dtb),96m(fs),-(user);spife110000.1:1m(uboot),5m(kernel),128k(dtb),-(user)"
 # CONFIG_CMD_IRQ is not set
 CONFIG_OF_CONTROL=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_ENV_ADDR=0xFFFC9000
 CONFIG_DM=y