macvlan interfaces never directly exchange traffic with the underlying
interface, but only with other hosts behind the interface. In consequence,
router advertisements from the uradvd running on br-client could never
reach local-node, preventing it from getting an IPv6 address without RAs
from an external radvd. Fix this be replacing the macvlan interface with
a veth pair (with the peer interface in br-client).

As a side effect, this saves about 5KB of flash, as the veth module is
simpler than macvlan.

When preparing the migration from macvlan to veth for local-node, MAC
address conflicts occurred as some ports of br-client had the same address
as local-node. Reverting the roles of both interfaces fixes this.

By default, br-client is left as an interface without addresses and
firewall rules that drop everything, so the bridge is used to connect its
ports only. gluon-mesh-batman-adv-core changes this to the usual set
of addresses and firewall rules.

Prepare reverting the roles of br-client and local-node to their state
before 8c4403ba.

Also simplify the local_node.peerdns setting.

Nothing outside the config mode depends on luci-base anymore.

We also make use of the boolean support of simple-uci to make scripts
clearer.

The maintenance overhead doesn't justify keeping this old stuff anymore.

Rework init script to additionally join a ff05::... multicast group on the mesh-wide interface.

Add missing trailing `,` in simple_tc section and change the order of ingress and egress to default format used by Gluon.

Fixes #1022

Fixes a build issue when switching targets.

Setting DEVICES will also suppress cleaning the package directory,
improving incremental build speed.

A lot of our code does not actually work with other prefix lengths.

Fixes #1014

Add gluon-dns-config: a package that enables the node to be used as DNS cache

The ar71xx-tiny target sets CONFIG_CLEAN_IPKG, which would delete opkg
control files required for user creation. Fix image generation and the user
creation script.

Fixes #1012

Fix an exception breaking the page.