rtos: abstraction: partition RTOS logic into separate directories

[lgirdwood]

In preparation for further RTOS abstraction, move the xtos and zephyr specific headers to new top level rtos directories to enable:

1. Partitioning of code by RTOS type e.g. xtos, Zephyr.
2. Partitioning of RTOS and non RTOS logic and APIs

This PR reuses the existing zephyr top level directory and creates a new xtos top level directory to store RTOS specific files, logic and wrappers. Other RTOSes like FreeRTOS or CMSIS support would be added in the same way.

This PR makes no runtime functional change, but changes some RTOS specific headers to be included with a new directory. i.e.

#include <sof/spinlock.h>

now becomes

#include <rtos/spinlock.h>

As this now maps to the correct RTOS and is no longer using the sof prefix which is for common code.

This PR also adds a high level Kconfig for developers (disabled by default) so that they can easily switch to fully native Zephyr headers for development testing.

Signed-off-by: Liam Girdwood [email protected]