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/at91rm9200ek_defconfig b/configs/at91rm9200ek_defconfig
index a990360..3b7d5b2 100644
--- a/configs/at91rm9200ek_defconfig
+++ b/configs/at91rm9200ek_defconfig
@@ -20,6 +20,7 @@
 CONFIG_CMD_MII=y
 CONFIG_CMD_PING=y
 CONFIG_CMD_FAT=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_ENV_ADDR=0x10040000
 CONFIG_AT91_GPIO=y