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/imx6ul_geam_nand_defconfig b/configs/imx6ul_geam_nand_defconfig
index dc27733..a48fa37 100644
--- a/configs/imx6ul_geam_nand_defconfig
+++ b/configs/imx6ul_geam_nand_defconfig
@@ -43,6 +43,7 @@
 CONFIG_MTDIDS_DEFAULT="nand0=gpmi-nand"
 CONFIG_MTDPARTS_DEFAULT="mtdparts=gpmi-nand:2m(spl),2m(uboot),1m(env),8m(kernel),1m(dtb),-(rootfs)"
 CONFIG_CMD_UBI=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_NAND=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_SYS_I2C_MXC=y