In some macOS configurations, macOS 10.13.5 on Macmini4,1 in my case,
rf2500hidapi transport fails to open RF2500 device with unknown
reason. This CL enables mspdebug to fallback to libusb backend in such
situation.
The transport/ subdirectory now contains implementations of the
transport_t interface, which has been expanded to include flush and
set_modem operations.