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/rocket.scala

@@ -27,8 +27,7 @@ case object FastJAL extends Field[Boolean]
 case object CoreInstBits extends Field[Int]
 case object NCustomMRWCSRs extends Field[Int]
 case object MtvecWritable extends Field[Boolean]
-case object MtvecInit extends Field[BigInt]
-case object ResetVector extends Field[BigInt]
+case object MtvecInit extends Field[Option[BigInt]]
 case object NBreakpoints extends Field[Int]
 case object NPerfCounters extends Field[Int]
 case object NPerfEvents extends Field[Int]