base-files: bridge config, avoid having a random ipv6 ll address (backport of r26426)

base-files: bridge config, avoid having a random ipv6 ll address (backport of r26426)

So far, we are setting the bridge interface up before having added any
bridge interface ports. This results in the bridge assigning a random
mac address to its bridge interface and therefore IPv6 assigning a
matching link local address to the bridge interface as soon as the
bridge interface is up. After adding the first bridge port interface,
the bridge's mac address is reset correctly, however the IPv6 link
local address stays the same.

This commit ensures that we are at least having the IPv6 link local
address of the first interface added to the bridge instead of a random
one.

Signed-off-by: Linus Lüssing <linus.luessing@web.de>
git-svn-id: svn://svn.openwrt.org/openwrt/branches/backfire@26427 3c298f89-4303-0410-b956-a3cf2f4a3e73