hostmot2: support boards on spi interface

.. for uspace systems with /dev/spidev.  This is mildly tested on odroid
u3 with a 7i43 custom firmware and with a real 7i90.

Note that on many systems out there, including odroid, /dev/spidev is lousy
for realtime performance.  Some kernel-side changes, available at
https://github.com/jepler/odroid-linux.git in branch odroid-3.8.13-rt
give good performance on my odroid u3 system.