frr: T5239: fix process startup order

• Reuse existing utility functions to check if a boot is ongoing (boot_configuration_complete())
• Run system_frr.py script to configure FRR daemon before initial launch
• Add safety net to always have FRR running on the system

This does yet not solve the error in T5239 but it's a small step towards
the solution.

(cherry picked from commit df74a09b80df0c2ec769a10ef4f7bac01f50eb2d)