tweaks needed for using only USB drive

[wjcarpenter]

Crankshaft NG has SD card partition names hard-coded in some places. That breaks booting and running from USB drives. For my own reasons (geometry), I need to skip the microSD card on my RPi 3B+. I tracked down all the things I found and needed to change, and I described them here: https://hackaday.io/page/395848-booting-crankshaft-ng-from-usb

[wjcarpenter]

Because of local build issues, I didn't see until today that the "deploy" directory contains a script "partition_setup.sh" that does the same sort of tweaks. AFAICT, that script is only referenced for the NOOBS version of the image. It ought to be possible to wire it into the vanilla image, possible as part of the RPi first-run stuff (for which Crankshaft has its own scheme).