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.

The MAV_TYPE enum was not in sync with the mavlink specs anymore. It
makes therefore sense to remove the duplication and include the correct
mavlink header file where it is needed.
Also, error counts which are not populated, can be scrapped.

Since the circuit breaker bools are not actually used anywhere else than
in the commander, it is safe to remove them and replace them with local
bools.

set default thrust scale parameter to zero - this will disable using the fw motor for mc pitch by default

in multirotor mode

implement moving forward using pusher

this causes only problems and is not supported by ANSI C. error caused:
fatal error: field '_buf' with variable sized type 'ubx_buf_t' not at the end of a struct or class is a GNU extension

this also fixes a compiler warning

the warning is:
error: ignoring return value of ‘ssize_t write(int, const void*, size_t)’, declared with attribute warn_unused_result [-Werror=unused-result]

- Note that the simulator still uses gpssim by default
- now the gps module can be used in the SITL. this makes it possible to test
  the real gps HW under POSIX
  additional steps needed to use it:
  - in the rcS_jmavsim_iris, make sure to start the gps instead of gpssim:
    gps start -d /dev/ttyACM0 -s
  - disable the mavlink serial connection in simulator_mavlink.cpp,
    openUart(PIXHAWK_DEVICE, 115200);
- this also fixes a memory leak in the gps module

when using ack to search the code we want to avoid matches in the doc html
files

this makes sure the header can also be used under POSIX