binary compatibility fix

Author: trmckay

core/src/main/scala/chisel3/BlackBox.scala

@@ -56,6 +56,9 @@ abstract class BlackBox(
   override protected final val requirements: Seq[String] = Seq.empty[String]
 ) extends BaseBlackBox {
 
+  def this(params: Map[String, Param], knownLayers: Seq[Layer]) =
+    this(params, knownLayers, Seq.empty[String])
+
   // Find a Record port named "io" for purposes of stripping the prefix
   private[chisel3] lazy val _io: Option[Record] =
     this

core/src/main/scala/chisel3/experimental/ExtModule.scala

@@ -100,6 +100,9 @@ abstract class ExtModule(
   override protected final val knownLayers:  Seq[Layer] = Seq.empty[Layer],
   override protected final val requirements: Seq[String] = Seq.empty[String]
 ) extends BaseBlackBox {
+  def this(params: Map[String, Param], knownLayers: Seq[Layer]) =
+    this(params, knownLayers, Seq.empty[String])
+
   private[chisel3] override def generateComponent(): Option[Component] = {
     require(!_closed, "Can't generate module more than once")