lib: add support for flavors in seg6 routes

lib: add support for flavors in seg6 routes

Until now, the 'show ipv6 route' was not showing up the flavors.

> ip/ip -6 route add 2007::1 encap seg6local action End flavors psp dev dum1

Before:
> rt1# show ipv6 route
> [..]
> K>* 2007::1/128 [0/1024] is directly connected, dum1, seg6local End -, weight 1, 00:00:04

After:
> rt1# show ipv6 route
> [..]
> K>* 2007::1/128 [0/1024] is directly connected, dum1, seg6local End (PSP), weight 1, 00:00:04

Note that the next-csid linux flavor will be handled separately.

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