Rework OverheadBase class (#375)

***NO_CI***

Author: josesimoes

nanoFramework.Tools.DebugLibrary.Shared/WireProtocol/Commands.cs

@@ -2029,7 +2029,15 @@ public static object ResolveCommandToPayload(uint cmd, bool fReply, CLRCapabilit
         public abstract class OverheadBase
         {
             [IgnoreDataMember]
-            public int Overhead { get; private set; }
+            public int Overhead
+            {
+                get
+                {
+                    return GetOverhead();
+                }
+
+                private set { }
+            }
 
             protected OverheadBase()
             {