Added accelerometer-triggered sleep

 -switchable at compile time with CONFIG_ACCEL_SLEEP flag
- maintain ring buffer of 3d acceleration readings (10 samples)
- compute RMS difference between last acceleration sample and buffered
readings (10 samples, 10 seconds)
- if difference is below a preset threshold for more than 5 minutes,
send the tag to sleep
- sleep is like hibernation, except we keep reading accelerometer
- if RMS difference exceeds threshold, wake up tag
- added sleep status to the flags field of the status packet