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.

* use tabs instead of spaces
* use jsonfilter for speed

Fixes #842

[Matthias Schiffer: changed the text a bit]