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/x530_defconfig b/configs/x530_defconfig
index 3911833..2e390b1 100644
--- a/configs/x530_defconfig
+++ b/configs/x530_defconfig
@@ -47,6 +47,7 @@
 CONFIG_CMD_FS_GENERIC=y
 CONFIG_CMD_MTDPARTS=y
 CONFIG_CMD_UBI=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_ADDR=0x100000
 CONFIG_SPL_OF_TRANSLATE=y
 CONFIG_BLK=y