!351 set default value for target_cpu

Merge pull request !351 from weichaox/master

hb_internal/preloader/preloader.py

@@ -239,8 +239,9 @@ class MyProduct():
             device_info['target_cpu'] = config['target_cpu']
         else:
             # Target cpu is used to set default toolchain for standard system.
-            print("The target_cpu needs to be specified.")
-            device_info['target_cpu'] = 'unknown'
+            print(
+                "The target_cpu needs to be specified, default target_cpu=arm")
+            device_info['target_cpu'] = 'arm'
         if config.get('kernel_version'):
             device_info['kernel_version'] = config.get('kernel_version')
         if config.get('device_build_path'):