This allows the spektrum and sbus receiver drivers to bind
directly to the usart layer using a properly exported API
rather than overriding the interrupt handler.
Bytes are now pushed directly from the usart layer into the
com layer without any buffering. The com layer performs all
of the buffering.
A further benefit from this approach is that we can put all
blocking/non-blocking behaviour into the COM layer and not
in the underlying drivers.
Misc related changes:
- Remove obsolete .handler field from irq configs
- Adapt all users of PIOS_COM_* functions to new API
- Fixup callers of PIOS_USB_HID_Init()
