imu: timestamping data

[gauteh]

• read timestamp from register and relate to now using RTC
• interleave timestamps in FIFO queue and compute a starting time for current batch

Or simpler:

• read offset in FIFO and set timestamp for start of batch

RTC has accuracy of 1/100th of sec (10ms = 10,000 us). timestamp on IMU has (numerical) accuracy of about 25 us.