Check which platform Silk is running on

[priyankaCh22]

TODO: Check what platform Silk is running on.
If it is an ARM device (e.g. raspberrypi), allow up to 2 minutes for wpantund to start.
if "armv7l" in subprocess.check_output("uname -m", shell=True):
self.wpantund_start_time = 120 # seconds
else:
self.wpantund_start_time = 30

https://github.com/openthread/silk//silk/node/fifteen_four_dev_board.py#L140