There's no build target, but what you CAN do...
Here, let me copy you my build script. :)
travis@Travis-PC(Gentoo) ~/building/dream-build $ cat build
zip update.zip -r META-INF boot.img data google.sh system
java -jar ../signing/javalib.jar ../signing/testkey.x509.pem ../
signing/testkey.pk8 update.zip traviseclair.zip
you need to use the jarsigning javalib.jar, it is somewhere in out/
host (or out/target/common... I honestly forgot) and use the two
testkeys. Maybe you can get away with other keys-- I just know using
jarsigner with my personal key didn't work. :(
META-INF is not needed, only if you want to run certain scripts. If
you go straight with using your system/ directory, isntead of
extracting the image there's nothing you need to repair (extracting
the image breaks a lot of symlinks, and that's what the purpose of the
META-INF directory before signing is). google.sh is a script I made,
and is not necessary for making an update.zip.