custom-devices: add timeout to ping command (#218)

This change adds 10sec timeout to ping command to discovery custom devices.

Signed-off-by: Hidenori Matsubayashi <Hidenori.Matsubayashi@gmail.com>

lib/elinux_device_discovery.dart

@@ -152,7 +152,7 @@ class ELinuxDeviceDiscovery extends PollingDeviceDiscovery {
       RunResult result;
       try {
         result = await _processUtils.run(remoteDevice.pingCommand,
-            throwOnError: true);
+            throwOnError: true, timeout: const Duration(seconds: 10));
         stdout = result.stdout.trim();
       } on ProcessException catch (ex) {
         _logger.printTrace('ping failed to list attached devices:\n$ex');