bgpd: Set NO_ADVERTISE community if blackhole community received

rfc7999:
A BGP speaker receiving an announcement tagged with the BLACKHOLE
community SHOULD add the NO_ADVERTISE or NO_EXPORT community as
defined in [RFC1997], or a similar community, to prevent propagation
of the prefix outside the local AS.  The community to prevent
propagation SHOULD be chosen according to the operator's routing
policy.

Sent:
```
router bgp 65534
 no bgp ebgp-requires-policy
 neighbor 192.168.0.2 remote-as 65030
 !
 address-family ipv4 unicast
  redistribute connected
  neighbor 192.168.0.2 route-map spine out
 exit-address-family
 !
!
ip prefix-list self seq 5 permit 192.168.100.1/32
!
route-map spine permit 10
 match ip address prefix-list self
 set community blackhole
!
```

Received:
```
spine1-debian-9# show ip bgp 192.168.100.1/32
BGP routing table entry for 192.168.100.1/32
Paths: (1 available, best #1, table default, inform peer to blackhole prefix)
  Not advertised to any peer
  65534
    192.168.0.1 from 192.168.0.1 (192.168.100.1)
      Origin incomplete, metric 0, valid, external, best (First path received)
      Community: blackhole no-advertise
      Last update: Thu Jan 21 12:56:39 2021
```

Signed-off-by: Donatas Abraitis <donatas.abraitis@gmail.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/bgpd/bgp_community.h b/bgpd/bgp_community.h
index b99f38ab64..2a1fbf526a 100644
--- a/bgpd/bgp_community.h
+++ b/bgpd/bgp_community.h
@@ -88,6 +88,7 @@ extern struct community *community_delete(struct community *,
 					  struct community *);
 extern struct community *community_dup(struct community *);
 extern bool community_include(struct community *, uint32_t);
+extern void community_add_val(struct community *com, uint32_t val);
 extern void community_del_val(struct community *, uint32_t *);
 extern unsigned long community_count(void);
 extern struct hash *community_hash(void);