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/grpeach_defconfig b/configs/grpeach_defconfig
index a76b8ec..0144e47 100644
--- a/configs/grpeach_defconfig
+++ b/configs/grpeach_defconfig
@@ -26,6 +26,7 @@
 CONFIG_CMD_FS_GENERIC=y
 CONFIG_MAC_PARTITION=y
 CONFIG_OF_CONTROL=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_USE_ENV_SPI_BUS=y
 CONFIG_ENV_SPI_BUS=0