Concept : 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 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.

Minor changes in mt.h,mt.c
Changed stack size for native linux

This pull request replaces #923