minimize stacksize of the idle thread

with this patch activated:

```
2013-09-25 15:28:40,807 - INFO # > ps
2013-09-25 15:28:40,816 - INFO # 	pid | name                 | state    Q | pri | stack ( used) location   | runtime | switches 
2013-09-25 15:28:40,824 - INFO # 	  0 | idle                 | pending  Q |  31 |   256 (  152) 0x40000010 |    nan% |       -1
```