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/MPC8541CDS_defconfig b/configs/MPC8541CDS_defconfig
index 5c25c4f..cd2d195 100644
--- a/configs/MPC8541CDS_defconfig
+++ b/configs/MPC8541CDS_defconfig
@@ -17,6 +17,7 @@
 CONFIG_CMD_MII=y
 CONFIG_CMD_PING=y
 # CONFIG_CMD_HASH is not set
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_ENV_ADDR=0xFFFC0000
 # CONFIG_MMC is not set