Replace occurences of ar71xx with ath79. Remove references of ar71xx
targets which are not relevant anymore.

The rewrite of the feature handling introduced multiple major bugs. One
of them was caused by the way Lua's logical operators work:

An expression of the form

    _'autoupdater' and _'web-advanced'

would return 'web-advanced' rather than the boolean true when _ returned
both strings unchanged (because the features are enabled).

As entries with more than a single feature name in their expressions did
not set no_default, Gluon would then attempt to add gluon-web-advanced to
the package selection, as web-advanced is a "pure" feature.

To fix this, and get rid of the annoying nodefault, separate handling of
"pure" feature and handling of logical expressions into two separate
functions, called feature() and when(). To simplify the feature
definitions, the package list is now passed directly to these functions
rather than in a table with a single field 'packages'.

Fixes: ee5ec5af ("build: rewrite features.sh in Lua")

The `features` file is converted to a Lua-based DSL.

A helper function `_` is used in the DSL; this will return the original
string for enabled features, and nil for disabled features. This allows
to use boolean operations on features without making the code too
verbose.

Besides having more readable and robust code, this also fixes the bug
that all files `packages/*/features` were evaluated instead of only
using the feature definitions of currently active feeds.

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>

This summaries giving an overview of a scripts function and a short summary
how it's doing this. Only the scripts are covered, that are used by the
Freifunk-Berlin firmwarebuiler too.

[Matthias Schiffer: slightly reworded some descriptions]

Closes #1993

Fixes commit 6692095f

Resolves #1929

The docs previously only listed ath9k and ath10k based chipsets as fully
supported by Gluon, however we also support mt76.

At the December 2019 review day, the behavior for LAN only devices was
defined. Such devices will use each LAN port as WAN.

$(TOPDIR) does not contain a trailing slash.
Thus the 'gluon.mk'-include must include
the trailing slash. (Just like the link to
$(INCLUDE_DIR)/package.mk does.)

Signed-off-by: Chris Fiege <chris@tinyhost.de>

small grammatical fix

the tag was added by dd23a805

Fixes #1491

Explain the magic of gluon.mk. The feature flag documention is moved into
this new page.

The group was renamed during the development of Gluon v2017.1, adjust the
documentation accordingly.

To reduce the number of packages that need to be listed in
GLUON_SITE_PACKAGES, this adds a new variable GLUON_FEATURES. Sets of
packages are enabled automatically based on the combination of listed
feature flags.

Site-specified package feeds can provide their own feature flag
definitions.

For simplicity, we don't use different MTUs for compat 14 and 15 anymore,
there's no harm in using 1532 for batman-adv-legacy as well.

_translate() will return nil when no match is found.