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