Tested build on a minimal Ubuntu 16.04 container image without wget.

The GLUON_ATH10K_MESH must be set to 11s or ibss; when it is not set,
ath10k device images won't be built at all. This also allows us to remove
the BROKEN flag for ath10k devices, as the GLUON_ATH10K_MESH variable is
sufficient to avoid ath10k devices if desired.

Fixes #864

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.

`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.

Build requires Python 2 and libssl-dev

noob-proofing a bit

add information about the environment variables.