[Bridge + 8021q] separated add/remove interface to/from bridge

Signed-off-by: Claudiu Ghioc <claudiu.ghioc@gmail.com>
[Bridge + 8021q] if_add: create a new interface for each VLAN

* if_add: if the new interface must be in "trunk" mode, we create
a new virtual interface for each VLAN in the switch;

Signed-off-by: Claudiu Ghioc <claudiu.ghioc@gmail.com>