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/sbc8641d_defconfig b/configs/sbc8641d_defconfig
index 67fbb1c..ea601de 100644
--- a/configs/sbc8641d_defconfig
+++ b/configs/sbc8641d_defconfig
@@ -18,6 +18,7 @@
 CONFIG_CMD_PING=y
 CONFIG_MP=y
 CONFIG_DOS_PARTITION=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_ADDR=0xFFF60000
 # CONFIG_MMC is not set
 CONFIG_MTD_NOR_FLASH=y