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/M52277EVB_defconfig b/configs/M52277EVB_defconfig
index 41db8bd..402ad7b 100644
--- a/configs/M52277EVB_defconfig
+++ b/configs/M52277EVB_defconfig
@@ -18,6 +18,7 @@
 CONFIG_CMD_CACHE=y
 CONFIG_CMD_DATE=y
 CONFIG_CMD_JFFS2=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_ENV_ADDR=0x40000
 # CONFIG_NET is not set