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/MigoR_defconfig b/configs/MigoR_defconfig
index 54012a3..af0c7d0a 100644
--- a/configs/MigoR_defconfig
+++ b/configs/MigoR_defconfig
@@ -24,6 +24,7 @@
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_CMD_PING=y
 # CONFIG_CMD_MISC is not set
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_ENV_ADDR=0xA0020000
 CONFIG_MTD_NOR_FLASH=y