This commit adds support for the new ipq40xx target.

In multidomain setups, VXLAN is enabled by default, but can be disabled in
domain configs using the mesh/vxlan option. In single domain setups, the
mesh/vxlan option is mandatory.

The UCI option for legacy mode is removed.

Fixes #1364

gluon-core, gluon-l3roamd: introduce script gluon-list-mesh-interfaces that lists all currently active mesh interfaces

dnsmasq's caching is severly broken and does not handle all answer records
equally. In particular, its cached answers are missing DNSKEY and DS
records, breaking DNSSEC validation on clients.

Remove the cache for now. It may return if dnsmasq is fixed or we switch to
a different resolver.

The script cleaned up an old setting that was removed in 2014.

net.ipv6.conf.br-client.forwarding is moved from gluon-client-bridge to
gluon-mesh-batman-adv, as the setting is not useful with non-bridged
protocols.

Our VXLAN setup was changed to accept VXLAN packets without checksum almost
2 months ago, so we can disable sending the checksums now as well. Slightly
improves performance.

exec() is moved to gluon.util.

* change type of next_node.name
* create domain entry for each entry and add to dnsmasq configuration

[Matthias Schiffer: reword documentation]

Both gluon.sysconfig and libgluonutil already remove the trailing newline
if it exists. It's nicer to avoid files without a trailing newline, e.g.
for printing the file contents in a terminal.

Patch originally authored by @oleeander.

Fixes #424

Allows to remove some redundant UCI lookups.

Fixes #1322

The prefix is not used, and requesting it leads to odhcp6c log spam with
certain DHCPv6 servers.

Based-on-patch-by: lemoer <git@irrelefant.net>