Previously the interaction between the different layers of the Bluetooth
stack was different on each port and each stack. This commit defines
common interfaces between them and implements them for cyw43, btstack,
nimble, stm32, unix.
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.
Jim Mussared