Concept : cooperative mt scheduler with events

- Non preemtive mt-scheduler (context switch during irq not allowed for this implementation)
- sending events from a contiki-process to a smt-thread works by  using smt_post in the contiki-process
- sending events from a smt-thread to a smt-thread works by using smt_post in the source smt_thread
- sending events from a smt-thread to a contiki-process works by using process_post / process_post_synch in the smt-thread
- smt-thread can be sent to bed with smt_sleep. This does only block the smt-thread calling sleep, not the whole system.
- an smt-thread has to exit on its own. Killing the thread from outside is not possible
- A broadcast of an event to all smt-threads can be achieved by setting the target thread to SMT_BROADCAST when invoking smt_post.
- Don't forget to call mt_init() and smt_init() before using this lib.
- polling threads from irq, main thread or other threads context by using smt_poll()
- pausing mt_threads, similar to PROCESS_PAUSE
- does not break current mt_thread implementation

Changed stack size for native linux

This pull request replaces #923