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/mccmon6_sd_defconfig b/configs/mccmon6_sd_defconfig
index 49fdc40..b78fa68 100644
--- a/configs/mccmon6_sd_defconfig
+++ b/configs/mccmon6_sd_defconfig
@@ -41,6 +41,7 @@
 CONFIG_MTDIDS_DEFAULT="nor0=8000000.nor"
 CONFIG_MTDPARTS_DEFAULT="mtdparts=8000000.nor:32m@0x0(mccmon6-image.nor),256k@0x40000(u-boot-env.nor),1m@0x80000(u-boot.nor),8m@0x180000(kernel.nor),8m@0x980000(swupdate-kernel.nor),8m@0x1180000(swupdate-rootfs.nor)"
 CONFIG_OF_CONTROL=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_SYS_REDUNDAND_ENVIRONMENT=y
 CONFIG_ENV_ADDR=0x8040000