This package allows to automatically switch to another domain, either
at a given point in time or after the node was offline long enough.

Gluon has multiple ways to obtain unique MAC-addresses. They are either
provided by the WiFi driver or derived from the primary MAC-address.

Quoting the same file:

> It's necessary that the first 45 bits of the MAC address don't
> vary on a single hardware interface, since some chips are using
> a hardware MAC filter. (e.g 'rt305x')

This currently fails in case the rt35xx based chips mac address differs
from the primary MAC. In this case, the MAC address for the client0 radio
(vif 1) comes from the WiFi driver. As there is only a single
MAC-address provided by '/sys/class/ieee80211/phyX/addresses' but the
MAC-address for mesh 0 (vif 2) is derived from the Node-ID, resulting in
different first 45 bits. The WiFi won't come up altogether in this case.

This commit verifies at least 4 MAC-Addresses are provided by the WiFi
driver. If this is not the case, all MAC-addresses are derived from the
primary MAC. This way, affected radios are working correctly.

gluon-core, gluon-mesh-babel: move lua functions that generate ll-addresses from a mac into gluon-core

exec() is moved to gluon.util.

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.

Allows to remove some redundant UCI lookups.

[Matthias Schiffer: rebase]

This should not convert JSON to a Lua table and back, as this loses the
distinction between arrays and objects, but as our site.conf is defined in
Lua anyways (for now), this can be fixed in a later revision.

[Matthias Schiffer: rename to gluon-show-site, rebase]

By basing the Lua gluon.site module on gluonutil_load_site_config(), the
config load implementation needs to changed only in a single place for
multi-domain support.

In particular, this affects users of gluon.util.iterate_radios.

The new gluon.site lua library will eventually replace gluon.site_config
(which is hereby deprecated, but will continue to be supported for a
while).

The new gluon.site library will wrap all values to allow traversing
non-existing tables without errors.

    site = require 'gluon.site'
    c = site.a.b.c -- doesn't fail even if a or a.b don't exist

The wrapped values must be unwrapped using call syntax:

    site_name = site.site_name()

Using the call syntax on a non-existing value will return nil. An
alternative default value may be passed instead:

    mac = site.next_node.mac('16:41:95:40:f7:dc')

While we use the hexadecimal representation as a hash input for simplicity,
it should not be interpreted as case-sensitive.

gluon-core: gluon.util: pass UCI cursor as argument to iterate_radios() instead of using local context

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

gluon-core: add get_mesh_devices(uconn) to gluon.util which will return all devices being up and having proto = gluon_mesh

Fixes #842

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.