system: T5505: T5575: support calling system-ip(v6).py from init process

After commit 976f82785 ("T5575: ARP/NDP table-size isnt set properly") the
system bootup process got interrupted as both system-ip.py and system-ipv6.py
tried to talk to FRR which was yet not started.

This has been fixed by using a conditional path to only execute when FRR service
has been enabled. This is safe to do as the initial commit call will has FRR
service running and the path will be executed.