Support SONiC debian8 on FRR 5.0.x

[zhenggen-xu]

- What I did Make FRR 5.0 buildable with SONiC debian8 dev environment.

- How I did it Add sonic.debian8 target with proper settings for SONiC

- How to verify it Build with sonic-buildimage (PR TBD), and load the image. FRR starts fine, and also FPM worked as expected towards SONiC :

$ vtysh

Hello, this is FRRouting (version 5.0.1). Copyright 1996-2005 Kunihiro Ishiguro, et al.

sonic# show running-config Building configuration...

Current configuration: ! frr version 5.0.1 frr defaults traditional hostname sonic log syslog informational log facility local4 ! router bgp 65100 bgp router-id 10.1.0.1 bgp log-neighbor-changes no bgp default ipv4-unicast bgp bestpath as-path multipath-relax neighbor 10.0.0.1 remote-as 65200 neighbor 10.0.0.1 description ARISTA01T2 neighbor 10.0.0.3 remote-as 65200 neighbor 10.0.0.3 description ARISTA02T2 neighbor 10.0.0.5 remote-as 65200 neighbor 10.0.0.5 description ARISTA03T2 neighbor 10.0.0.7 remote-as 65200

....

sonic# configure terminal sonic(config)# ip route 10.10.10.0/24 10.0.0.17

redis-cli:

127.0.0.1:6379> hgetall ROUTE_TABLE:10.10.10.0/24

1. "nexthop"
2. "10.0.0.17"
3. "ifname"
4. "Ethernet8"

More tests will be done at sonic-buildimage PR.

- Description for the changelog

[lguohan]

is this still needed?