make a zedboot USB key
usage: fx mkzedboot [-f] [--build|--no-build] <usb device> --build | --no-build build (or not) the image required to run mkzedboot. -f force writing to a non-usb target. The default for --build is defined by the "incremental" feature: 'fx --enable=incremental mkzedboot' defaults to '--build' 'fx --disable=incremental mkzedboot' defaults to '--no-build'