Android build environment

Android build environment

I have been working with standard android for some time. Recently I have
been building android sdk for windows and find some interesting things in
Android build env. There is a command called lunch which can be used to
list all available targets to build. However, it does not list sdk and its
variants,even though they are valid options. Does anyone know why is that
the case?
Also can I build any specific module in Android sdk without building the
whole sdk. (It could be a great help as I dont wanna build the whole sdk,
if I gonna do a small change in one component.)
Thanks