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.