An idea of adding all-in-one Safety Status Display

> Cloned from diydrones/ardupilot#2171

At present, the Pre-Arm safety check only return errors via text messages every 30 sec or arming. We could only receive one error every time.

I'd like to add a new mavlink message to represent the safety status in 1Hz, including all checks' results. Like this:

``` xml

For SAFETY_STATUS.

RC not calibrated


Baro not healthy


Alt disparity


Compass not healthy


Compass not calibrated


Compass offsets too high


Check mag field


compasses inconsistent


check fence


INS not calibrated


Accels not healthy


Accels inconsistent


Gyros not healthy


Gyro cal failed


Gyros inconsistent


Check Board Voltage

and more....

Add soon.
Add soon.

```

And in `static bool pre_arm_checks(bool display_failure)`, change them like this

``` cpp
pre_arm_rc_checks();
if(!ap.pre_arm_rc_check) {
// if (display_failure) {
// gcs_send_text_P(SEVERITY_HIGH,PSTR("PreArm: RC not calibrated"));
safetystatus += SAFETY_STATUS_FUNC.PREARM_STATUS_FUNC_RC;
}
// return false;
flag = false;
}
```

In MP, add a UserControl like this:

May it affect the performence, or have other bad effects?

Thanks.

Start by tracing the `static bool pre_arm_checks(bool display_failure)` entry point and how Mission Planner renders the proposed UserControl. Review the requested SAFETY_STATUS message and all listed pre-arm checks before deciding the message and display boundaries. Done would require an agreed design for reporting all check results at 1Hz without adverse performance effects.

Written by the indexing model from the issue text.