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/am3517_crane_defconfig b/configs/am3517_crane_defconfig
index 6d181fa..8ad2c84 100644
--- a/configs/am3517_crane_defconfig
+++ b/configs/am3517_crane_defconfig
@@ -24,6 +24,7 @@
 CONFIG_CMD_EXT2=y
 CONFIG_CMD_FAT=y
 CONFIG_CMD_JFFS2=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_NAND=y
 CONFIG_MMC_OMAP_HS=y
 CONFIG_MTD=y