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/phycore-am335x-r2-wega_defconfig b/configs/phycore-am335x-r2-wega_defconfig
index e1739cc..f6f2dad 100644
--- a/configs/phycore-am335x-r2-wega_defconfig
+++ b/configs/phycore-am335x-r2-wega_defconfig
@@ -48,6 +48,7 @@
 CONFIG_DOS_PARTITION=y
 CONFIG_OF_CONTROL=y
 CONFIG_SPL_OF_CONTROL=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_FAT=y
 CONFIG_ENV_IS_IN_NAND=y
 CONFIG_ENV_FAT_DEVICE_AND_PART="0:1"