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/maxbcm_defconfig b/configs/maxbcm_defconfig
index a98c329..e443a15 100644
--- a/configs/maxbcm_defconfig
+++ b/configs/maxbcm_defconfig
@@ -32,6 +32,7 @@
 CONFIG_CMD_MII=y
 CONFIG_CMD_PING=y
 CONFIG_CMD_TIME=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_USE_ENV_SPI_MAX_HZ=y
 CONFIG_ENV_SPI_MAX_HZ=50000000
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y