commander Preflight::preflightCheck: transition condition_system_sensors_initialized only to true

- this matches the logic in arming_state_transition()
- if Commander::preflight_check was run in armed state and the checks
  failed, disarming was not possible anymore
  (because disarming checks for condition_system_sensors_initialized).
  That is currently not the case, but the existing logic is way too fragile.

Alternative solution: check if armed in Preflight::preflightCheck, and
only transition to false if disarmed (but also respect the re-arming
grace period).