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/p3450-0000_defconfig b/configs/p3450-0000_defconfig
index 51bfd05..e37463e 100644
--- a/configs/p3450-0000_defconfig
+++ b/configs/p3450-0000_defconfig
@@ -27,6 +27,7 @@
 # CONFIG_CMD_NFS is not set
 CONFIG_CMD_EXT4_WRITE=y
 CONFIG_OF_LIVE=y
+CONFIG_ENV_OVERWRITE=y
 # CONFIG_ENV_IS_IN_MMC is not set
 CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_DFU_MMC=y