HAL_Linux: only init serial(0)

we should not pre-init all uarts, it wastes memory if unused

libraries/AP_HAL_Linux/HAL_Linux_Class.cpp

@@ -377,12 +377,7 @@ void HAL_Linux::run(int argc, char* const argv[], Callbacks* callbacks) const
     gpio->init();
     rcout->init();
     rcin->init();
-    uartA->begin(115200);
-    uartE->begin(115200);
-    uartF->begin(115200);
-    uartG->begin(115200);
-    uartH->begin(115200);
-    uartI->begin(115200);
+    serial(0)->begin(115200);
     analogin->init();
     utilInstance.init(argc+gopt.optind-1, &argv[gopt.optind-1]);