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/lager_defconfig b/configs/lager_defconfig
index 69026f7..c91a516 100644
--- a/configs/lager_defconfig
+++ b/configs/lager_defconfig
@@ -57,6 +57,7 @@
 CONFIG_MTDIDS_DEFAULT="nor0=spi0.0"
 CONFIG_MTDPARTS_DEFAULT="mtdparts=spi0.0:256k(u-boot-spl),512k(u-boot-env1),512k(u-boot-env2),768k(u-boot),-(user)"
 CONFIG_OF_CONTROL=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_ENV_ADDR=0xC0000
 CONFIG_CLK=y