Supervisors/UAV/LostComms: Check if lost comms plan exists before issuing PlanControl start request.
Supervisors/UAV/LostComms: Once PlanDb exists BOOT, check if lost comms plan exists.
Merge branch 'feature/uav-lost_comms-check_plan'

Bug Fix #3177: Lost comms plan is now only issued if it is defined in the
PlanDB. This prevents multiple new idle plans/logs from being generated
due to failed attempts to start non-existing lost comms plans.

Authored By: José Braga
Reviewed By: Pedro Calado

This commit fixes #3177.