Start with a clean build (this will take a long time):
$ sudo ./build.sh
When complete, turn off rebuilds of the intermediate stages:
touch stage{0,1,2}/SKIP piscan/SKIP stage2/SKIP_IMAGES piscan/SKIP_IMAGES
Then we can iterate on rebuilding the last stage (piscan-2) quickly with:
sudo CLEAN=1 ./build.sh
Completed builds are in deploy/image_`date +%F`-pi-scan-finalfinal.zip