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/apx4devkit_defconfig b/configs/apx4devkit_defconfig
index 32ca8b2..aa81445 100644
--- a/configs/apx4devkit_defconfig
+++ b/configs/apx4devkit_defconfig
@@ -33,6 +33,7 @@
 CONFIG_MTDIDS_DEFAULT="nand0=gpmi-nand"
 CONFIG_MTDPARTS_DEFAULT="mtdparts=gpmi-nand:128k(bootstrap),1024k(boot),768k(env),-(root)"
 CONFIG_CMD_UBI=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_NAND=y
 CONFIG_SYS_REDUNDAND_ENVIRONMENT=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y