add SERVOS to blackbox optional fields list

[stronnag]

Prior behaviour: If servos were defined in the mixer, servos were always logged (all servos before INAV8, defined servos INAV8+). In most cases, servo logging was not needed.
New Behaviour: 'SERVOS' has been added as an option to the CLI blackbox list. By default SERVOS are enabled for logging. The user can disable SERVO logging with blackbox -SERVOS (or a TBD Configurator entry).

Usage

# blackbox 
Enabled: NAV_POS NAV_PID MAG ACC ATTI RC_DATA RC_COMMAND MOTORS SERVOS

For a flying wing generates BBL headers ... motor[0],servo[0],servo[1],navStat ...

# blackbox -SERVOS
Disabled SERVOS
# blackbox
Enabled: NAV_POS NAV_PID MAG ACC ATTI RC_DATA RC_COMMAND MOTORS 

For the same FW results in ... motor[0],navState ...

[stronnag]

Note that the PG_VERSION has been bumped to enforce default settings. Users will need to reapply customised settings.

[stronnag]

Configurator will need to be updated (and the SITL FILE option should also be added to the same page). To follow.