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/sh7763rdp_defconfig b/configs/sh7763rdp_defconfig
index 9c5acf4..7c9b7cc 100644
--- a/configs/sh7763rdp_defconfig
+++ b/configs/sh7763rdp_defconfig
@@ -28,6 +28,7 @@
 CONFIG_CMD_PING=y
 # CONFIG_CMD_MISC is not set
 CONFIG_CMD_JFFS2=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_SYS_REDUNDAND_ENVIRONMENT=y
 CONFIG_ENV_ADDR=0xA0020000