Initialize the rtimer component on native platform's startup.

Otherwise applications that use rtimers die when run as native code because of an unhandled signal.
Make native platform's main loop less noisy.
Explicitly initialize queue buffer in the main startup routine only if uIP is used.

The Rime stack already does this on its own, and there is no need to do it twice.
Merge pull request #656 from atiselsts/native_platform_patches

Minor improvements for the native platform