The implementation of SYS_FLEN was missing, which caused picolibc to
treat this file as not implemented. Additionally, there was a bug in
the SYS_READ call that was comparing the wrong variable against the
passed buffer length. It was comparing the current file position against
the buffer length instead of the number of written bytes.
Finally, pos was unititialized which could result in spurious errors.
Change-Id: I8b487a79df5970a5001d3fef08d5579bb4aa0dd0
bb4c13143c