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'