Implement the bwfm(4) SDIO bus logic. This is the bus layer that

Implement the bwfm(4) SDIO bus logic.  This is the bus layer that
converts the logic of the upper layers (sending control messages,
sending data messages, receiving event or data messages) into the
corresponding work that has to be done on the lowest layer.  SDIO
is not the fastest bus for exchanging network packets, but maybe
there is room for tuning.  Actual TX/RX is being done in a worker
task that serializes access to the hardware.  This is good enough
to attach to WiFi networks and do network transfers.  Developed
and tested on a Cubox-i.