CC2420 timing diff with hardware

[simonduq]

There is a timing difference between a real cc2420 and MSPSim: the time between a call to NETSTACK_RADIO.transmit() (with the radio OFF) and the end of the SFD is, on hardware, about 456 usec. In MSPSim it is 369 usec.

What is needed is to add an extra 87 usec delay in MSPSim before every transmission.