This adds documentation for the gluon-mesh-batman-adv package and
elaborates on its build and configuration options, as well as
the implemented multicast architecture.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>

Issue was uncovered by sphinx-build:
docs/user/getting_started.rst:100: WARNING: Definition list ends without a blank line; unexpected unindent.

This new build flag is mandatory for now (it may default to 0 in a future
Gluon version). It may be set to the following values:

* 0 - Do not build any images for deprecated devices.
* upgrade - Only build sysupgrade images for deprecated devices.
* full - Build both sysupgrade and factory images for deprecated devices.

"Other" images are handled like factory images, as they are also used for
the initial installation of Gluon on a device.

Add the `wifi5.outdoor_chanlist` site configuration that
allows specifying an outdoor channel range that can be
switched to for regulatory compliance.

Upon enabling the outdoor option the device will
 - configure the `outdoor_chanlist` on all 5 GHz radios
 - which may enable DFS/TPC, based on the regulatory domain
 - disable ibss/mesh on the 5 GHz radio, as DFS *will*
   break mesh connections
 - allow for htmode reconfiguration on 5 GHz radios

The outdoor option can be toggled from
 - Advanced Settings
   - W-LAN
     - Outdoor Installation

The `preserve_channel` flag overrules the outdoor channel
selection.

A table with hardware supported by Gluon can be found on the OpenWrt Wiki. This commit adds a corresponding FAQ entry.

I just followed this document on a Debian Stretch, and it worked out of
the box. So let's use Debian's current stable release here.

Before 7827f896, mandatory hostname field in config mode was
pre-filled with the default hostname.

This commit adds the config_mode.hostname.prefill option for
controlling the default value.

gluon-config-mode-geo-location-osm extends the
gluon-config-mode-geo-location with a location picker based on
OpenStreetMaps.

Based-on-patch-by: Jan-Tarek Butt <tarek@ring0.de>

As already done with other config mode texts, the altitude field now has
default texts that are used when they are not set in the site i18n files.
The altitude-help text has been removed from site i18n; instead, the
geo-location-help text now overrides the whole section description
including the part that mentions the altitude.