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/mx7ulp_com_defconfig b/configs/mx7ulp_com_defconfig
index 7eb9deb..dd596bf 100644
--- a/configs/mx7ulp_com_defconfig
+++ b/configs/mx7ulp_com_defconfig
@@ -23,6 +23,7 @@
 CONFIG_CMD_CACHE=y
 CONFIG_CMD_FAT=y
 CONFIG_OF_CONTROL=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_MMC=y
 # CONFIG_NET is not set
 CONFIG_DM=y