Accessor for device ID.

A command to the nugget app returns the device ID. An option is added to
citadel_updater to fetch the ID.

Test: citadel_updater --id
Change-Id: I9fec7f029de14b79039956efe2d73e9245148717
diff --git a/nugget/include/app_nugget.h b/nugget/include/app_nugget.h
index c6acfc0..aa8a798 100644
--- a/nugget/include/app_nugget.h
+++ b/nugget/include/app_nugget.h
@@ -156,6 +156,16 @@
  * @errors             APP_ERROR_BOGUS_ARGS
  */
 
+#define NUGGET_PARAM_DEVICE_ID 0x0006
+/*
+ * Get the device ID of the chip.
+ *
+ * @param args         <none>
+ * @param arg_len      0
+ * @param reply        Null-terminated ASCII string
+ * @param reply_len    Max length to return
+ */
+
 
 /****************************************************************************/
 /* Test related commands */