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/MPC8572DS_defconfig b/configs/MPC8572DS_defconfig
index fea1e28..5733458 100644
--- a/configs/MPC8572DS_defconfig
+++ b/configs/MPC8572DS_defconfig
@@ -23,6 +23,7 @@
 CONFIG_MP=y
 # CONFIG_CMD_HASH is not set
 CONFIG_CMD_EXT2=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_ADDR=0xEFF20000
 CONFIG_SCSI_AHCI=y
 CONFIG_SYS_FSL_DDR2=y