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/MPC8610HPCD_defconfig b/configs/MPC8610HPCD_defconfig
index 9879823..b9ef566 100644
--- a/configs/MPC8610HPCD_defconfig
+++ b/configs/MPC8610HPCD_defconfig
@@ -22,6 +22,7 @@
 CONFIG_CMD_BMP=y
 CONFIG_CMD_EXT2=y
 CONFIG_DOS_PARTITION=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_ADDR=0xFFF80000
 CONFIG_SCSI_AHCI=y
 # CONFIG_MMC is not set