Fix log pull

I noticed trying to pull logs with ArduCopter 3.2 was mostly unreliable. Changed the line handling so it waits until it sees the first line that *should* be in a log file before starting to collect lines to write to the log file, and keeps collecting until it sees the "Logs]" line. This seems hack-ish and perhaps there's a more deterministic way to read logs, but this is the best I came up with. Please try it and verify it works for you. Thanks.