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/bcm7445_defconfig b/configs/bcm7445_defconfig
index 65c4b33..edf299e 100644
--- a/configs/bcm7445_defconfig
+++ b/configs/bcm7445_defconfig
@@ -24,6 +24,7 @@
 CONFIG_CMD_EXT4=y
 CONFIG_CMD_FS_GENERIC=y
 CONFIG_OF_PRIOR_STAGE=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_SYS_REDUNDAND_ENVIRONMENT=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y