Put the prebuild openvpn binary to "assets/openvpn" then:
# under root of android source tree
source build/envsetup.sh
breakfast `your device model`
cd `OpenVpn folder`
mm
Put your signkey at OpenVpn folder/signkey.keystore (optional)
export ANDROID_NDK_HOME=`your ndk path`
export ANDROID_HOME=`your sdk path`
mvn install # or with -Djarsigner.storepass=KEYPASS