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/r7780mp_defconfig b/configs/r7780mp_defconfig
index daacd43..44bf3a7 100644
--- a/configs/r7780mp_defconfig
+++ b/configs/r7780mp_defconfig
@@ -29,6 +29,7 @@
 # CONFIG_CMD_MISC is not set
 CONFIG_CMD_EXT2=y
 CONFIG_DOS_PARTITION=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_ENV_ADDR=0xA0040000
 CONFIG_MTD_NOR_FLASH=y