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/env/Kconfig b/env/Kconfig
index 5784136..4113628 100644
--- a/env/Kconfig
+++ b/env/Kconfig
@@ -6,6 +6,12 @@
 config SAVEENV
 	def_bool y if CMD_SAVEENV
 
+config ENV_OVERWRITE
+	bool "Enable overwriting environment"
+	help
+	  Use this to permit overriding of certain environmental variables
+	  like Ethernet and Serial
+
 config ENV_IS_NOWHERE
 	bool "Environment is not stored"
 	default y if !ENV_IS_IN_EEPROM && !ENV_IS_IN_EXT4 && \