snabb-softwire-v2.yang: declare softwire/b4-ipv6 unique
snabb-softwire-v2.yang: declare softwire/b4-ipv6 unique

b4-ipv6 is used as the key for binding-entires in the ietf-softwire-br model,
so for the models to be translatable b4-ipv6 must be unique.

Igalia/snabbswitch,alexandergall/snabbswitch,snabbco/snabb,eugeneia/snabb,Igalia/snabbswitch,snabbco/snabb,alexandergall/snabbswitch,SnabbCo/snabbswitch,Igalia/snabbswitch,Igalia/snabb,eugeneia/snabb,Igalia/snabb,snabbco/snabb,SnabbCo/snabbswitch,snabbco/snabb,Igalia/snabb,snabbco/snabb,eugeneia/snabb,alexandergall/snabbswitch,Igalia/snabb,snabbco/snabb,SnabbCo/snabbswitch,Igalia/snabb,snabbco/snabb,alexandergall/snabbswitch,Igalia/snabb,alexandergall/snabbswitch,SnabbCo/snabbswitch,alexandergall/snabbswitch,Igalia/snabbswitch,eugeneia/snabb,eugeneia/snabb,snabbco/snabb,Igalia/snabb,Igalia/snabb,eugeneia/snabb,alexandergall/snabbswitch,eugeneia/snabb,Igalia/snabbswitch,eugeneia/snabb,alexandergall/snabbswitch