core/hwtimer: disable interrupts in hwtimer_remove

Before only the hardware timer's own interrupt was being disabled.
This led to a race condition in the following scenario:

```
Thread1:
    hwtimer_remove()
    hwtimer_arch_disable_interrupt();

// INTERRUPT -> Thread2 (which has a higher priority than Thread1) gets scheduled

Thread2:
    ...
    hwtimer_remove()
    hwtimer_arch_disable_interrupt(); // hwtimer interrupt is already disabled
    ...
    hwtimer_arch_enable_interrupt();
    ...
   // yield | terminate -> Thread1 gets scheduled again

Thread1:
    ... // these instructions are being run with the hwtimer interrupt enabled
    hwtimer_arch_enable_interrupt(); // hwtimer interrupt is already enabled
```

Fixes #924

core/hwtimer.c

@@ -202,14 +202,16 @@ int hwtimer_set_absolute(unsigned long offset, void (*callback)(void*), void *pt
 int hwtimer_remove(int n)
 {
     DEBUG("hwtimer_remove n=%d\n", n);
-    hwtimer_arch_disable_interrupt();
+
+    int state = disableIRQ();
     hwtimer_arch_unset(n);
 
     lifo_insert(lifo, n);
     timer[n].callback = NULL;
 
     lpm_prevent_sleep--;
 
-    hwtimer_arch_enable_interrupt();
+    restoreIRQ(state);
+
     return 1;
 }