碰上一个写在init.rc里面的service不能自动启动的问题，加上class好了。
service yourservice /sbin/yourservice
class core
user adb
group adb
class <name>
Specify a class name for the service. All services in a
named class may be started or stopped together. A service
is in the class "default" if one is not specified via the
class option.
看起来是因为
All services in a named class may be started or stopped together.
而如果default的class都没有起来的话，这个service也就不会自动启动了
所以 class_start default. 还是有用的....
Ref:
[Android init language]
http://www.netmite.com/android/mydroid/1.6/system/core/init/readme.txt