How to add a new program to the external folder of Android

1. Create a folder named "myprog" in external folder.
2. Create two files,Android.mk and main.c.
     Example:
     Android.mk
    ===============================
    LOCAL_PATH:=$(call my-dir)

   include $(CLEAR_VARS)

   LOCAL_SRC_FILES:= /
                                            main.c

   LOCAL_MODULE:= myprog

   LOCAL_MODULE_TAGS:=optional

   include $(BUILD_EXECUTABLE)
   ================================
   main.c
   ================================
   #include <stdio.h>
   void main(void)
  {
       printf("Hello World!!");
  }

3. The all Android.mk will be all scanned before source compilation so that the LOCAL_MODULE_TAGS is important.

4. LOCAL_MODULE_TAGS is related with TARGET_BUILD_VARIANT.

    The new program is definitely built up when TARGET_BUILD_VARIANT is equal to LOCAL_MODULE_TAGS.
     For example, presume the several lunch methods.
     -- lunch testplatform-userdebug
     -- lunch testplatform-user
     -- lunch testplatform-eng
  After lunch-ing them,TARGET_BUILD_VARIANT can be set to "userdebug","user" and "eng".The program will be definitely built with
    -- LOCAL_MODULE_TAGS := debug
    --LOCAL_MODULE_TAGS :=user
    --LOCAL_MODULE_TAGS:=eng

5.why needs LOCAL_MODULE_TAGS := optional

   LOCAL_MODULE_TAGS is set to optional can indicate system compilier to check make file for the build. The new program can be add to package build at the Android source/device/testplatform/xxx.mk. The "curl" can be used for the example of package add in xxx.mk. If the new program is not added in package build of makefile,system won't compiler the new program.