[sonic-installer] Fix an issue that old config is not preserved in aboot image upgrade (sonic-net#122)

Author: taoyl-ms

sonic_installer/main.py

@@ -154,8 +154,8 @@ def install(url):
     else:
         os.chmod(image_path, stat.S_IXUSR)
         run_command(image_path)
-        run_command("cp -r /etc/sonic /host/old_config")
         run_command('grub-set-default --boot-directory=' + HOST_PATH + ' 0')
+    run_command("cp -r /etc/sonic /host/old_config")
     click.echo('Done')