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/mx51evk_defconfig b/configs/mx51evk_defconfig
index a65f020..07c5653 100644
--- a/configs/mx51evk_defconfig
+++ b/configs/mx51evk_defconfig
@@ -22,6 +22,7 @@
 CONFIG_CMD_PING=y
 CONFIG_CMD_DATE=y
 CONFIG_CMD_FAT=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_MMC=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_FSL_ESDHC_IMX=y