Generate a signature file on boot

Currently, FlySight devices can be recognized by inspecting their USB
IDs. This changeset causes the FlySight to automatically generate a
FLYSIGHT.TXT file containing a unique identifier. This allows simple,
user-level tools to identify that a) a newly-mounted volume contains
FlySight data, and b) this data was generated by a specific unit.
Merge branch 'signature_file'