WIP pi-gen build of Pi Scan

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
$ touch piscan/SKIP
$ touch stage2/SKIP_IMAGES
$ touch 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