(cherry picked from commit bf8c3bab)

This change stores a Kernel with Debug-Symbols for the current
architecture in a new output directory '<outputdir>/debug'.
This allows a developer or operator of a network to store the kernel
along with the actual images. In case of a kernel oops the debug
information can be used with the script
'scripts/decode_stacktrace.sh' in the kernel source tree to get the
names to the symbols of the stack trace.

OpenWRT already provides the CONFIG_COLLECT_KERNEL_DEBUG -option that
creates a kernel with debug-symbols in the OpenWRT output directory.
This change enables this option and copies the generated kernel to the
gluon output directory.

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

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.