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]

Signed-off-by: Sven Eckelmann <sven.eckelmann@open-mesh.com>

As the Gluon kernel won't be compatible with the OpenWrt upstream anymore,
we don't need the vermagic hack anymore.

The file targets/$GLUON_TARGET/config becomes optional, as many targets
only used it to set the board and subtarget.

Also fix targets without subtarget.

With a backported patch from the OpenWrt trunk, it is now easy to generate an
equivalent configuration using CONFIG_ALL_KMODS.

The build will take a bit longer because all kernel module packages are actually
built even when they are not included in the image, but adding new targets
becomes a lot easier.

Also, related documentation updates and fixes.