Split osal into interface library and implementation

[elupus]

Expose a osal-interface target that is of cmake INTERFACE type. This target only defines the osal API and allow delaying the choice of linking of final osal port implementation to the link stage of the binary. This allow the Osal implementation to be defined by the integrator of used libraries.

To accomplish this:

• osal.h becomes static and common to all targets
• osal_sys.h is changed to an internal implementation header for the osal port
• Buld files are restructured to keep build configuration of a port in the port directory

By default an osal cmake alias is added to remain compatible with previous solutions.