Previously, the Cooja mote assumed that its file was always initially empty (file.endptr == 0). Therefore, a file uploaded to a mote's CFS could never be read by the mote, as the mote would prevent reads from going past the EOF (indicated by endptr).
By tracking the file size and making it accessible to Cooja, the correct size of the uploaded file can be reported to the mote and allow it to read the uploaded file.
This change means that one needs to select _ONE_ cfs implementation at Contiki library link time. But this doesn't appear to be an issue as all platforms have their "favorite" implementation anyway.
