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.

With all the packages being built 10GB are not enough anymore.

fixes an error and removes the focus on ath10k

defaults to 11s, allowing to build devices without IBSS-support

obsoletes GLUON_ATH10K_MESH

Adding the fact that GLUON_RELEASE is needed with 'make manifest'
if it is used with 'make'.

This fact was mentioned in the v2017.1 release notes but is not (clearly)
mentioned inside other parts of the documentation.

Signed-off-by: Chrissi^ <chris@tinyhost.de>

[Matthias Schiffer: minor rewording]

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