Convert CONFIG_CMD_ENV_CALLBACK to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_ENV_CALLBACK

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index 6b9feff..3ac8764 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -314,6 +314,14 @@
 	  Check if a variable is defined in the environment for use in
 	  shell scripting.
 
+config CMD_ENV_CALLBACK
+	bool "env callbacks - print callbacks and their associated variables"
+	help
+	  Some environment variable have callbacks defined by
+	  U_BOOT_ENV_CALLBACK. These are called when the variable changes.
+	  For example changing "baudrate" adjust the serial baud rate. This
+	  command lists the currently defined callbacks.
+
 endmenu
 
 menu "Memory commands"