Link PlusArg to emulator command line options

- adds a mutable singleton (PlusArgArtefacts) to store information
  about Rocket PlusArgs
- adds methods to PlusArgArtefacts to emit C snippets that are
  consumed by emulator.cc for correct argument parsing and help text
  generation
- emits snippets in $(CONFIG).plusArgs via BaseCoreplex-set
  ElaborationArtefacts
- modify emulator/Makefrag-verilator to include $(CONFIG).plusArgs
- cleanup help text (docstring) for existing PlusArgs

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@gmail.com>

src/main/scala/coreplex/BaseCoreplex.scala

@@ -21,6 +21,7 @@ abstract class BareCoreplexModule[+L <: BareCoreplex](_outer: L) extends LazyMod
   ElaborationArtefacts.add("graphml", outer.graphML)
   ElaborationArtefacts.add("dts", outer.dts)
   ElaborationArtefacts.add("json", outer.json)
+  ElaborationArtefacts.add("plusArgs", PlusArgArtefacts.serialize)
   println(outer.dts)
 }