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/sh7753evb_defconfig b/configs/sh7753evb_defconfig
index b1563ed..1461ba0 100644
--- a/configs/sh7753evb_defconfig
+++ b/configs/sh7753evb_defconfig
@@ -27,6 +27,7 @@
 CONFIG_CMD_EXT2=y
 CONFIG_MAC_PARTITION=y
 CONFIG_DOS_PARTITION=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_MMC=y
 CONFIG_SH_MMCIF=y
 CONFIG_BITBANGMII=y