Signed-off-by: David Bauer <mail@david-bauer.net>

(cherry picked from commit 7f3a3042)

(cherry picked from commit 447595f0)

fixes #1935
found by @lrnzo

(cherry picked from commit 63ebeb25)

(cherry picked from commit 239c379d)

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.

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