changes to SG file schema to simplify later processing

[jbrzusto]

• include serno, ts, bootcount etc. in a record in first line of each file, rather than relying on filenames
  being preserved (which, courtesy of DOS-Fat32 weirdness of some intermediate memory devices,
  they are not always)
• write a random token at the end of one file and the same token at the beginning of the next, so that
  we can easily reconstruct the sequence of files even when names are scrambled and the system
  clocks has not been synced to GPS time.