Transition to using bus_driver for SPI

[ncasaril]

Currently a semaphore in the bsp is used top avoid collisions on the spi bus used for the dwX000 and hal_spi_XX calls used to access the bus.

• What needs to be changed in bsp's to support the bus_driver_spi?
• What needs to change in the dwX000_hal?
• Would this allow us to simplify the linux / mynewt interfacing currently in use in dwX000_hal.c?

[szymon-czapracki]

@ncasaril Is there an option to unassign me from this :) (doing some spring cleanups :P )