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/MPC8349ITX_LOWBOOT_defconfig b/configs/MPC8349ITX_LOWBOOT_defconfig
index a1d2a89..46f7afc 100644
--- a/configs/MPC8349ITX_LOWBOOT_defconfig
+++ b/configs/MPC8349ITX_LOWBOOT_defconfig
@@ -170,6 +170,7 @@
 CONFIG_CMD_DATE=y
 CONFIG_CMD_EXT2=y
 CONFIG_CMD_FAT=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_ADDR=0xFE080000
 CONFIG_SATA_SIL3114=y
 # CONFIG_MMC is not set