Fix thread_yield() on MSP430 platforms

Avoid race conditions that can cause crash on MSP430 platforms, when interruptions prevent thread context switching from being correctlty done.

It fixes the biggest cause of failure for the hwtimer_wait test on MSP430 (see issue #426), the other cause being fixed by #1619.
According to the coding conventions (this is "inherited" by the Linux coding style) multi-line comments should be formated:
```
/*
 * <stuff>
 */
```