compile-fc-batt: allow possible third field in source lines Battery tables maintained in the fc-battery-conf repository will now have a third field added, defining thresholds for the battery bars icon, and there will be a new utility to compile them into the new /etc/batterytab2 file read by the FC Tourmaline version of our FCHG driver. For backward compatibility with the original Magnetite version of FCHG, compile-fc-batt remains the tool for compiling the original /etc/batterytab file format, and it needs to ignore the newly added third field in battery table sources.