Sketching out device driver subsystem to allow extensible device management functions to be made available to applications.
Revising some implementations and adding tests.
Removing fingerprint and extension concept for now.

Change-Id: Id50faf29e4590b08e0c2b385ae8d4bb0c453f37e
diff --git a/core/api/src/main/java/org/onosproject/net/DefaultAnnotations.java b/core/api/src/main/java/org/onosproject/net/DefaultAnnotations.java
index 514152e..a0ed261 100644
--- a/core/api/src/main/java/org/onosproject/net/DefaultAnnotations.java
+++ b/core/api/src/main/java/org/onosproject/net/DefaultAnnotations.java
@@ -29,6 +29,8 @@
  */
 public final class DefaultAnnotations implements SparseAnnotations {
 
+    public static final Annotations EMPTY = DefaultAnnotations.builder().build();
+
     private final Map<String, String> map;
 
     // For serialization