boards: Fix CC2420 RXFIFO overflow

Attempts to implement CC2420 ISR to handle RXFIFO overflow compliant to datasheet.

Contains changes for:
* z1
* telosb
* wsn430-v1_4

fixes #315 
I think this is only true if both interrupts are pending. Shouldn't it be rather:
```
if (((P1IN & CC2420_FIFOP_PIN) != 0) && (P1IFG & CC2420_GIO0_PIN) != 0)
```
-> GIO0 goes low, while FIFOP is still high?