gluon-mesh-batman-adv-core: keep disabled state even when the mesh interface section name is changed

Is makes sense to always look for both ibss_radio* and mesh_radio* sections
to determine if the meshing should be enabled when regenerating these
sections. Doing this, the disabled state will survive updates changing the
section name (either updating from pre-2015.2 while keeping IBSS, or
changing from IBSS to 11s or vice-versa).

If both ibss_radio* and mesh_radio* sections exist, the disabled state will
be kept correctly for each section, the behaviour is changed only when
creating a section that didn't exist before.

Fixes #549

PKG_FILE_DEPENDS caused the whole site repo to be hashed to determine if
gluon-site needed rebuilding. While this is normally no problem,
alternative build setups sometimes put the Gluon repo inside the site repo,
causing long build times and parallel build failures.

docs: improve user/getting_started, some minor fixes in user/site

Fixes #550

This commit basically does two things: it urges users always to use an own
git repository to manage the site configuration, and it removed all
mentions of "communities" from the "Getting started" page, as it was too
Freifunk-specific.

Other documentation pages (especially the site configuration docs) should
probably be adjusted as well.

Confusion of WAN an LAN, this line is for mesh-on-lan

These devices are identical to the TL-WR941 v4 & v5, so we don't need to
test them explicitly.

usign has pulled in new dependencies, and OpenWrt doesn't like the command
`make tools/prepare` in a clean build.

Fixes #540

This will fix the early preparation happening much too often and also fixes
early preparation after `make clean`.

Update kernel to 3.18.23

This broke due to the Makefile changes for building the module repo.

Update site.rst

The Freifunk Münsterland firmware was formerly known as Freifunk Münster firmware - while the URL hasn't changed. In future, there will probably be subdomains with their own files, but for now this is state of the project.

doc: remove duplicate article in getting_started.rst

Generate opkg repo for kernel modules

`make modules` will generate a opkg repository containing kernel modules
matching the kernel of the Gluon images.

We'll soon have other output besides the images, having a single directory
for all output makes sense.

The upstream builds always use LINUX_RELEASE=1. By setting it to 2 we
ensure that our modules are always preferred as long as our kernel version
is at least as high as the upstream one.

As the Gluon kernel won't be compatible with the OpenWrt upstream anymore,
we don't need the vermagic hack anymore.

doc: remove double opening next_node line in site.conf example

I guess this is some copy and paste mistake which makes the example an
invalid site.conf. In my case merging the changes of the example into
our actual site.conf lead to an error because this line was merged to.

Fixes #376

Fixes #534