给apk系统签名的方法

以Guard为例，在系统代码的编译环境下，把Guard.apk放到pacakges/apps/Guard目录下，编写Android.mk（如下），编译此目录mmm pacakges/apps/Guard，或者加入项目编译：

# Android.mk

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

# Module name should match apk name to be installed.
LOCAL_MODULE := Guard
LOCAL_SRC_FILES := $(LOCAL_MODULE).apk
LOCAL_MODULE_TAGS := optional
LOCAL_MODULE_CLASS := APPS
LOCAL_MODULE_SUFFIX := $(COMMON_ANDROID_PACKAGE_SUFFIX)
LOCAL_CERTIFICATE := platform
include $(BUILD_PREBUILT)