After repo sync (when you got make files). - after this step:

cd ~/aosp || { echo " ~/aosp not exists :)"; exit 1; }
repo init -u https://android.googlesource.com/platform/manifest -b android-15.0.0_r1
repo sync

Try to do this command in aosp folder.

. build/envsetup.sh
export TARGET_RELEASE=ap2a  

if ap2a doesnt work try to ap3a ,then

build_build_var_cache 
lunch

Now you got full list of Android builds.