[FRR]: Aligned configs for unified/separated modes.

[nazariig]

Signed-off-by: Nazarii Hnydyn [email protected]

- What I did

• Aligned FRR configs for unified/separated modes
• Fixed IPv6 neighbors issue: [frr] prefers ipv6 link-local as next hop address #2986

- How I did it

• Fixed bgpd.conf.j2

- How to verify it

root@sonic:/home/admin# ndisc6 fc00::76 PortChannel0002
Soliciting fc00::76 (fc00::76) on PortChannel0002...
Target link-layer address: 52:54:00:F6:F6:4A
 from fc00::76

root@sonic:/home/admin# ndisc6 fe80::34f1:92ff:feb9:129c PortChannel0002
Soliciting fe80::34f1:92ff:feb9:129c (fe80::34f1:92ff:feb9:129c) on PortChannel0002...
Target link-layer address: 36:F1:92:B9:12:9C
 from fe80::34f1:92ff:feb9:129c

- Description for the changelog

• N/A