As a partial fix to #496, do not touch the MAC address of the WAN
interface when using VXLANs (as only the MAC address of the VXLAN
interface matters to batman-adv).

For simplicity, we don't use different MTUs for compat 14 and 15 anymore,
there's no harm in using 1532 for batman-adv-legacy as well.

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

gluon-client-bridge, gluon-mesh-batman-adv-core: switch roles of br-client and local-node interfaces

MAC and IP addresses are switched. This makes the gluon-client-bridge
package more useful for different routing protocols that don't need a
unique address on the client bridge.

As a side effect, gluon-radvd is now using the next-node address, which had
been considered before, but was dismissed to avoid having gluon-radvd
depend on gluon-next-node and gluon-mesh-batman-adv. This will be useful
for announcing default routes via gluon-radvd.

One downside is that this introduces a minor dependency on batman-adv in
gluon-respondd: the hotplug script that checked for the client interface
before will now check for local-node. This doesn't really matter: for mesh
protocols without a local-node interface, the check will do nothing (which
makes sense, as there is no interface to bind to for mesh-wide respondd).

Fixes #842

Switch to:

1. WAN
2. LAN
3. Mesh VPN

As WAN and LAN are setup in gluon-mesh-batman-adv-core (and will be moved
to gluon-core), while the mesh VPN has its own package, giving WAN and LAN
the first indices is preferable.

Some drivers (mt76) don't support arbitrary MAC addresses. Use the
addresses provided by the driver (avoiding the primary address) by default,
but fall back to our has-based scheme when the driver doesn't provide
(enough) addresses.

While ath9k/ath10k devices can supprt VIFs with any combination of MAC addresses, there are also adapters which have a hardware MAC filter which only allows a few bits to differ. This commit changes the addresses of all VIFs to ony differ in the last 3 bits, which is required to support many Ralink/Mediatek based WLAN adapters.

Technically, the new addresses are generated by calculating an MD5 hash of the primary MAC address and using a part of this hash as a prefix for the MAC addresses.

The addresses (BSSIDs) of the AP VIFs are also reused for the LAN and WAN interfaces in mesh-on-LAN/WAN mode to reduce the number of needed addresses, and thus reduce the chance of collisions. This is not a problem as the MAC addresses of the AP VIFs are never used except as BSSID, and thus not seen by routing protocols like batman-adv.

Fixes #648

[Matthias Schiffer: rewrote commit message]

Avoid commiting configs inbetween to decrease number of flash writes.

Fixes #714.

This fixes MAC address conflicts on devices which use the eth0 as primary
address.

Moving the scripts to a common directory not only vastly simplifies the
zzz-gluon-upgrade script, but also allows to define an ordering of such
scripts across packages.

Also move it to gluon-mesh-batman-adv, as mesh_on_wan is the only feature that
needs a unique MAC address on the WAN interface.