Android比较重要的三个img文件：
- make systemimage - system.img
- make userdataimage - userdata.img
- make ramdisk - ramdisk.img
- make snod - 快速打包system.img (with this command, it will build a new system.img very quickly. well, you cannot use “make snod” for all the situations. it would not check the dependences. if you change some code in the framework which will effect other applications)
大家可以使用命令 make showcommands snod 来看看make snod将要执行的命令的具体内容，比如
make snod: ignoring dependencies Target system fs image: out/target/product/jasmine/system.img out/host/linux-x86/bin/mkyaffs2image -f out/target/product/jasmine/system out/target/product/jasmine/system.img true