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/sbc8349_defconfig b/configs/sbc8349_defconfig
index 2b06272..b69e66b 100644
--- a/configs/sbc8349_defconfig
+++ b/configs/sbc8349_defconfig
@@ -78,6 +78,7 @@
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_CMD_MII=y
 CONFIG_CMD_PING=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_SYS_REDUNDAND_ENVIRONMENT=y
 CONFIG_ENV_ADDR=0xFF840000
 CONFIG_ENV_ADDR_REDUND=0xFF860000