When trying to move the selected power source to the first publication instance
on systems where there is only one power source, the compiler issued the warning
```
../src/modules/sensors/sensors.cpp:512:54: error: array subscript is outside array bounds [-Werror=array-bounds]
tmp_h = _battery_pub[_battery_pub_intance0ndx];
```
because it could not verify that _battery_pub_intance0ndx would always be 0.
Wrap the block between `#if BOARD_NUMBER_BRICKS > 1 [...] #endif`
to ensure no out of bound subscript and to remove the warning.
Remove unused _battery_pub_intance0ndx variable when nb bricks = 1
2c0539ae9c