Allow reset vector to be set dynamically

A chip's power-up sequence, or awake-from-sleep sequence, may wish to
set the reset PC based upon dynamic properties, e.g., the settings of
external pins.  Support this by passing the reset vector to the Coreplex.
ExampleTop simply hard-wires the reset vector, as was the case before.

Additionally, allow MTVEC to *not* be reset.  In most cases, including
riscv-tests, pk, and bbl, overriding MTVEC is one of the first things
that the boot sequence does.  So the reset value is superfluous.

src/main/scala/rocket/frontend.scala

@@ -35,6 +35,7 @@ class Frontend(implicit p: Parameters) extends CoreModule()(p) with HasL1CachePa
     val cpu = new FrontendIO().flip
     val ptw = new TLBPTWIO()
     val mem = new ClientUncachedTileLinkIO
+    val resetVector = UInt(INPUT, vaddrBitsExtended)
   }
 
   val icache = Module(new ICache(latency = 2))
@@ -45,7 +46,7 @@ class Frontend(implicit p: Parameters) extends CoreModule()(p) with HasL1CachePa
   val s1_speculative = Reg(Bool())
   val s1_same_block = Reg(Bool())
   val s2_valid = Reg(init=Bool(true))
-  val s2_pc = Reg(init=UInt(p(ResetVector)))
+  val s2_pc = Reg(init=io.resetVector)
   val s2_btb_resp_valid = Reg(init=Bool(false))
   val s2_btb_resp_bits = Reg(new BTBResp)
   val s2_xcpt_if = Reg(init=Bool(false))