BL_HELI: Reverse Thrust ESC RPM reporting positive

### Bug report

RPM is reported as positive even during reverse thrust is active. Although I believe this may be a limitation of telemetry ESC's, I think ardupilot should switch the sign and log RPM as negative during reverse throttle application to the ESC.

This has applications in all vehicles - rovers driving backwards, copters flying upside down, planes with reverse thrust, subs, etc.

**Version**

Plane 3.10-latest roughly 2018-12-02

**Platform**
[X] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ ] Submarine

**Airframe type**
Believer

**Hardware type**
Lumineer 51A BlHeli_32 Telemetry ESC's

**Logs**
https://www.dropbox.com/s/x3lk1ezq49lagsr/00000018.BIN?dl=0

Start by examining the attached 00000018.BIN log and tracing how BLHeli_32 ESC telemetry RPM is handled when reverse throttle is active across the supported vehicle types. Done means RPM is reported and logged as negative during reverse thrust while remaining correct for forward operation.

Written by the indexing model from the issue text.