But only when bluetooth is enabled, i.e. if building the dev or coverage
variants, and we have libusb available.
Update travis to match, i.e. specify the variant when doing
`make submodules`.
This commit adds full support to the unix port for Bluetooth using the
common extmod/modbluetooth Python bindings. This uses the libusb HCI
transport, which supports many common USB BT adaptors.
