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/marsboard_defconfig b/configs/marsboard_defconfig
index 2dee097..1d9b621 100644
--- a/configs/marsboard_defconfig
+++ b/configs/marsboard_defconfig
@@ -21,6 +21,7 @@
 CONFIG_CMD_USB=y
 CONFIG_CMD_CACHE=y
 CONFIG_CMD_EXT4_WRITE=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_DM=y