HAL_Linux: fixed SPI semaphore, and panic on CS error

This adds a check for trying to assert two CS pins on the same bus
at the same time. The change involves moving the _device handles into the
DeviceManager class, and accessing via static methods.

This also moves the semaphore to be per-bus rather than per-device,
which fixes the problem with bad MS5611 transfers.

Pair-Programmed-With: Victor, Sid, Anuj and Philip
Linux_HAL: added dts files for overlays

These are the BBB/PXF specific DTS files, fixed for the SPI pin handling
Linux_HAL: updated binary dtbo files
HAL_Linux: added basic support for RC overrides

this allows for easier testing of RC output by allowing
mavproxy to setup fake inputs

Pair-Programmed-With: Sid, Anuj, Victor and Philip