Batt_usedmah2 Value needed in MP >> Flight Data Screen

#### Issue details
_Please describe the problem, or desired feature_

#### Version
MP 1.3.49 build 1.1.6410.20232

Batt_usedmah2 required
================

Hi Michael Oborne

I am using 2 Power Modules for PH2.1 , ACv3.5.2 for redundancy and calculating their currents.

Instead there is batt_usedmah for the First Power Module and i am using it

but i need value batt_usedmah2 for monitoring 2nd Battery Capacity in flight.

Hope you look into this.

Cheers
Ray Robert

Start at Mission Planner's Flight Data screen and locate where the existing batt_usedmah value is displayed. Trace the corresponding battery telemetry handling, then expose batt_usedmah2 for the second power module and verify that the second battery capacity appears during flight.

Written by the indexing model from the issue text.