This commit adds support for the new ipq40xx target.

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

Adds support for Alfa AP121F USB powered travel router (looks like an USB ethernet adapter).
There is no factory image, but only a sysupgrade file for this router which can be installed using the inbuilt UBoot web interface.
Configure your PC with IP address 192.168.1.2 and connect the router using its RJ45 port. Hold the reset button while powering the device until all LEDs have flashed 3 times simultaneously. You can now access the fallback web interface using a web browser at http://192.168.1.1 and flash the sysupgrade file.

based on package documentation, authored by T_X 
https://github.com/freifunk-gluon/gluon/blob/84a6f65f02d7e36a073ba2839712f0c0bb1dda10/package/gluon-ebtables-limit-arp/Makefile#L18-L39

fixes #1383

dnsmasq's caching is severly broken and does not handle all answer records
equally. In particular, its cached answers are missing DNSKEY and DS
records, breaking DNSSEC validation on clients.

Remove the cache for now. It may return if dnsmasq is fixed or we switch to
a different resolver.

At the moment, we don't have a good guideline for package-specific
configuration, but it seems like a good idea not to split configuration
into too many tiny pages, especially for packages that aren't commonly
selected explicitly.

Some uncommon configuration is dropped from the example site.conf to remove
clutter.

We must ensure that each node becomes IGMP/MLD querier for its local
clients; having only a single querier for the whole mesh is generally
unreliable, leading to frequent "IGMP/MLD querier appeared/disappeared"
messages from batman-adv and unreliable snooping.

In smaller meshes it might be interesting only segment querier domains, but
allow membership reports to pass through the mesh, in order to support
snooping switches outside the mesh without special configuration. A
site.conf switch is provided to control this behaviour.

Fixes #1320

[Matthias Schiffer: slightly clean up code]

This package drops all incoming router advertisements except for the
default router with the best metric according to B.A.T.M.A.N. advanced.

Note that advertisements originating from the node itself (for example
via gluon-radvd) are not affected.

defaults to 11s, allowing to build devices without IBSS-support

obsoletes GLUON_ATH10K_MESH

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.