Add debug modes to blackbox tab

[IvoFPV]

Fixes #1502
Adds debug mode setting in the blackbox tab. Requires MSP to go with it which will be added.

[McGiverGim]

@hydra is that ok for you now?

no response

[hydra]

sorry for late reply on this one @McGiverGim been a busy week yes, that is perfect now.

I did note in another PR recently that the internal debug variables in multiwii/baseflight/cleanflight/betaflight all pre-date blackbox logging and are for developers.

The variables are not specifically for logging, but by adding this to the UI on the blackbox logging page you might think they were for that when this is not the case.

Renaming the setting 'Blackbox debug mode' further confuses this situation.

debug variables are used in the following places:

1. internally in firmware
2. readable via msp commands
3. displayable on OSD.
4. on 'sensors' page in the configurator UI.
5. in the blackbox log data.

For a long time I thought about adding this setting to the 'sensors' page in the UI but never got round to it. Perhaps this setting should be moved from the 'Blackbox' page to the 'Sensors' tab, or perhaps it really belongs in the catch-all 'Configuration' page and called 'Debug mode' rather than 'Blackbox debug mode'.

Perhaps the real solution is to allow the user to choose exactly what they want to log in the 4 blackbox log uint16_t placeholders from all available sources and leave 'debug mode' alone.