LOCAL_MODULE_TAGS的这几个选项意思

"Set LOCAL_MODULE_TAGS to any number of whitespace-separated tags.

This variable controls what build flavors the package gets included
in. For example:

  * user: include this in user/userdebug builds 
  * eng: include this in eng builds
  * tests: the target is a testing target and makes it available for
tests
  * optional: don't include this"

Are these the same as "variants" and if so, which name would affect
the build and how? I've noticed that everything mentioned in a
product's makefile will always get built. But what gets in the final
system.img not always the same as what gets built.
以下为可能性最大的答案：
就是mk的标记啊，

直接给你上官方的吧

http://android.git.kernel.org/?p=platform/build.git;a=blob_plain;f=core/build-system.html;h=43bae03b6b7b9cba678b86d2faf424fa565497bf;hb=HEAD
  user:指该模块只在user版本下才编译
  eng: 指该模块只在eng版本下才编译
  tests: 指该模块只在tests版本下才编译
  optional:指该模块在所有版本下都编译