Plusargs -- tilelink timeout detection from the command line (#752)

* util: PlusArg gives Chisel access to the command-line

* tilelink2: add a progress watchdog to Monitors

vsrc/plusarg_reader.v

@@ -0,0 +1,22 @@
+// See LICENSE.SiFive for license details.
+
+// No default parameter values are intended, nor does IEEE 1800-2012 require them (clause A.2.4 param_assignment),
+// but Incisive demands them. These default values should never be used.
+module plusarg_reader #(string FORMAT="borked", int DEFAULT=0) (
+   output [31:0] out
+);
+
+reg [31:0] myplus;
+assign out = myplus;
+
+initial begin
+   myplus = DEFAULT;
+`ifndef SYNTHESIS
+`ifndef verilator
+   // Work-around for https://www.veripool.org/issues/1165
+   if (!$value$plusargs(FORMAT, myplus)) myplus = DEFAULT;
+`endif
+`endif
+end
+
+endmodule