Avoid duplicates when uploading with import_motes_zip

[jdavid]

For example, frames uploaded through Iridium will create metadata including the tags device_type, device_serial and imei. When uploading the data from the SD card this information is missing, so it will upload the same frames again (with different metadata).

Note that in the case of Iridium manual upload is required since only a subset of the frames is sent through the network.

This is vaguely related to issue #7