Artefact output (#545)

* build: stop using empty .prm file

* generator: general-purpose mechanism for creating elaboration artefacts

src/main/scala/rocketchip/BaseTop.scala

@@ -13,7 +13,7 @@ import util._
 import rocket._
 
 abstract class BareTop(implicit p: Parameters) extends LazyModule {
-  TopModule.contents = Some(this)
+  ElaborationArtefacts.add("graphml", graphML)
 }
 
 abstract class BareTopBundle[+L <: BareTop](_outer: L) extends GenericParameterizedBundle(_outer) {

src/main/scala/rocketchip/Generator.scala

@@ -77,7 +77,5 @@ object Generator extends util.GeneratorApp {
   val longName = names.topModuleProject + "." + names.configs
   generateFirrtl
   generateTestSuiteMakefrags
-  generateConfigString
-  generateGraphML
-  generateParameterDump
+  generateArtefacts
 }