bgpd: add L2 attr community support as per RFC8214

bgpd: add L2 attr community support as per RFC8214

The L2 attribute extended community can not be decoded when using L2VPN
EVPN as a route reflector. Decode the extended community and dump the
detailed information about flags and MTU information.

> rt4# show bgp l2vpn evpn
> BGP table version is 1, local router ID is 4.4.4.4
> Status codes: s suppressed, d damped, h history, * valid, > best, i - internal
> Origin codes: i - IGP, e - EGP, ? - incomplete
> EVPN type-1 prefix: [1]:[EthTag]:[ESI]:[IPlen]:[VTEP-IP]:[Frag-id]
> EVPN type-2 prefix: [2]:[EthTag]:[MAClen]:[MAC]:[IPlen]:[IP]
> EVPN type-3 prefix: [3]:[EthTag]:[IPlen]:[OrigIP]
> EVPN type-4 prefix: [4]:[ESI]:[IPlen]:[OrigIP]
> EVPN type-5 prefix: [5]:[EthTag]:[IPlen]:[IP]
>
>    Network          Next Hop            Metric LocPrf Weight Path
> Route Distinguisher: 1.1.1.1:100
>  *>i[1]:[12]:[00:00:00:00:00:00:00:00:00:00]:[32]:[0.0.0.0]:[0]
>                     1.1.1.1                       100      0 i
>                     RT:65500:100 L2: P flag:N, B Flag N, C word N, MTU 0
> Route Distinguisher: 5.5.5.5:100
>  *>i[1]:[10]:[00:00:00:00:00:00:00:00:00:00]:[32]:[0.0.0.0]:[0]
>                     5.5.5.5                       100      0 i
>                     RT:65500:100 L2: P flag:N, B Flag N, C word N, MTU 0
>

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>