Add a default baud rate of 9600

This will stop ARMmbed/mbed-drivers#128 from breaking test jobs until mbedgt can extract the default baud from yotta config.

Author: bremoran

target.json

@@ -23,6 +23,11 @@
     "mbed"
   ],
   "config": {
+      "mbed-os" : {
+          "stdio" : {
+              "default-baud" : 9600
+          }
+      },
     "minar": {
       "initial_event_pool_size": 50,
       "additional_event_pools_size": 100