A low battery check was recently added to the preflight check, therefore
we need to pass on the information about the battery.

This state is only commander internal. Therefore it doesn't need to be
in vehicle_status. Instead it is now in the commander_state message.

Instead of the state use the boolean flags.

All the removed flags were not used anywhere else than inside the
commander.

The offboard status bools were not used anywhere but in the commander.
Therefore they are now moved to the local status_flags topic.

Most condition bools in the commander are not used anywhere but in the
commander. It therefore makes sense to move them to a different internal
struct and remove them from the vehicle_status message.

Also, the land_detected should be used by all the modules instead of
getting it through the commander and system_status.

The name was set NULL which lead to a segfault on POSIX.

This flag is not used anywhere, it therefore doesn't need to be in
vehicle_status.

The counter variable in system status wasn't used anywhere.

The commander used to consume the battery_status topic and write the
contents after some calculations into the system state. Instead, the
calculations now happen in library calls in systemlib/battery.

This moves the battery fields out of the vehicle_status message into the
battery_status topic.

This brought quite some changes in all modules that need battery
information. The current state is compiling but untested.

The battery warning is not consumed anywhere, therefore scrap it from
the vehicle_status message.

It makes more sense to have the VTOL status in its own message.