Add bridge support on the image

Setting up an lxd cluster and found out there's no bridge support. Easy to add, painful to recompile.

``` bash
$ diff newconfig oldconfig
742,744c742
< CONFIG_STP=y
< CONFIG_BRIDGE=y
< CONFIG_BRIDGE_IGMP_SNOOPING=y

---
> # CONFIG_BRIDGE is not set
748d745
< CONFIG_LLC=y
```
