copy a file to/from a target device
usage: fx cp [-r] [--to-target|--to-host] SRC... DST
Copies a file from the host to the target device, or vice versa.
-r: recursively copy directories
--to-target: copy file SRC from host to DST on the target
--to-host: copy file SRC from target to DST on the host
The default is to copy files to the target.