#3250 breaks Blackbox GPS altitude

[stronnag]

#3250 naively changed the recording of altitude such that it could be negative. While we need to be able to record -ve altitudes, this is obviously NOT the correct solution, as it causes the recorded altitude to oscillate between +ve and -ve values, as the image below shows.

This affect has been observed on multiple vehicles, so its not just a stupid GPS. Code needs to be updated to use blackboxWriteSignedVB also.

Fix under test.

[stronnag]

Fixed by #3501