librem5dev: Fix kernel flavor and add usb host mode

This fixes an artifact from renaming the device from 'purism-librem5' to 'purism-librem5dev' and also sets the dts to one that enables usb host mode. Usb host mode is useful(essential?) on the devkit for adding a keyboard/mouse to interact with the desktop since the onboard display and touchscreen are very much in flux.