Flight Data tuning page option for SCALED_PRESSURE2 values

Some of the new flight controllers (like the Cube Black) have more than one barometer. Also it's now possible to add an external barometer even for a Pixhawk1 ([wiki](http://ardupilot.org/copter/docs/common-baro-external.html)).

It would make testing easier if the 2nd baro's pressure and temperature values (i.e. raw_press2, raw_temp2) was also visible in the Tuning and Quick screens.

The pressure and temperature for the 2nd barometer are sent in the [SCALED_PRESSURE2 message](http://mavlink.org/messages/common#SCALED_PRESSURE2).

Start by locating the Flight Data Tuning and Quick screens and the code that handles MAVLink SCALED_PRESSURE messages. Trace how raw_press2 and raw_temp2 could reach those displays; done means the second barometer’s pressure and temperature values are visible in both screens.

Written by the indexing model from the issue text.