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/ds414_defconfig b/configs/ds414_defconfig
index 55a793b..bf27ab5 100644
--- a/configs/ds414_defconfig
+++ b/configs/ds414_defconfig
@@ -43,6 +43,7 @@
 CONFIG_CMD_MTDPARTS=y
 CONFIG_CMD_UBI=y
 CONFIG_ISO_PARTITION=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_USE_ENV_SPI_MAX_HZ=y
 CONFIG_ENV_SPI_MAX_HZ=50000000
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y