benchmarks/examples: give comm_serial a device

These examples/benchmarks require manual handholding in
many cases anyway. Also, for comm_tcp manual changes are required.

Instead of nursing a bunch of different pdb and ddbs,
we probably want to force the user to copy and edit a template
that we then gitignore.

benchmarks/ddb.pyon

@@ -3,7 +3,7 @@
         "type": "local",
         "module": "artiq.coredevice.comm_serial",
         "class": "Comm",
-        "arguments": {}
+        "arguments": {"serial_dev": "/dev/ttyUSB1"}
     },
     "core": {
         "type": "local",

examples/master/ddb.pyon

@@ -3,7 +3,7 @@
         "type": "local",
         "module": "artiq.coredevice.comm_serial",
         "class": "Comm",
-        "arguments": {}
+        "arguments": {"serial_dev": "/dev/ttyUSB1"}
     },
     "core": {
         "type": "local",