provide OSAL fallback implementations
OSAL currently expects several "advanced" features (such as an event system or periodic timers) to be supported by the operating system.
Since most RTOSes do not support such out-of-the-box, OSAL should provide fallback implementation, using more rudimentary features (e.g. thread signaling and one-shot timers). Even though these fallbacks will probably perform worse than implementation integrated in the OS, this would provide a ways to get started with µRT quickly with any OS, which supports a fundamental feature set.