Fix the race condition when an interruption occured at the beginning

of thread_yield(), i.e.: just after SR(R2) was pushed but before
the rest of the suspended thread's context was pushed!

cpu/msp430-common/cpu.c

@@ -22,12 +22,12 @@ char __isr_stack[MSP430_ISR_STACK_SIZE];
    of its execution, in inlined __restore_context() sub-function */
 __attribute__((naked)) void thread_yield(void)
 {
-    __save_context();
-
     /* disable IRQ, remembering if they are
        to be reactivated after context switch */
     unsigned int irqen = disableIRQ();
 
+    __save_context();
+
     /* have sched_active_thread point to the next thread */
     sched_run();