java_gen: value types: add NO_MASK, FULL_MASK constants where missing

This is necessary so types can distinguish non-canonical forms.
diff --git a/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/U8.java b/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/U8.java
index 078a846..c644599 100644
--- a/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/U8.java
+++ b/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/U8.java
@@ -29,6 +29,10 @@
     private final static byte ZERO_VAL = 0;
     public final static U8 ZERO = new U8(ZERO_VAL);
 
+    private static final byte NO_MASK_VAL = (byte) 0xFF;
+    public static final U8 NO_MASK = new U8(NO_MASK_VAL);
+    public static final U8 FULL_MASK = ZERO;
+
     private final byte raw;
 
     private U8(byte raw) {
@@ -38,6 +42,8 @@
     public static final U8 of(short value) {
         if(value == ZERO_VAL)
             return ZERO;
+        if(value == NO_MASK_VAL)
+            return NO_MASK;
 
         return new U8(t(value));
     }