Fixed temperature for batteries

[AmeliaEScott]

Describe problem solved by the proposed pull request
The current implementation for batteries does not measure temperature, so it was reporting the temperature as 0C.

Test data / coverage
Used the MAVLink console in QGC to verify that the temperature is correctly reported as INT16_MAX.

Describe your preferred solution
Changed internal UOrb battery_status message to use NAN to represent unknown temperature. Changed mavlink_messages.cpp to correctly convert this to INT16_MAX as per the MAVLink message spec.

[AmeliaEScott]

I had some trouble with isnan(...). The "Compile All Boards" CI check would pass, but then "SITL Tests" would fail to build because it could not find the symbol isnan. I implemented my own isnan, but I don't know if that is a good solution.

This problem happened when using std::isnan from cmath, as well.

[dagar]

Try PX4_ISFINITE() instead.