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.

The next-node MAC address doesn't need to be unique in different
communities, so we can as well add a default value.

The SSID for the mesh network should be cryptic, otherwise some users will try to connect to that network and have a bad user experience.

The fastd_mesh_vpn site.conf section is renamed to mesh_vpn.fastd.

This was introduced in ffhh-packages at https://github.com/freifunkhamburg/ffhh-packages/tree/master/gluon-config-mode-reboot-ffhh

Tested build on a minimal Ubuntu 16.04 container image without wget.

The GLUON_ATH10K_MESH must be set to 11s or ibss; when it is not set,
ath10k device images won't be built at all. This also allows us to remove
the BROKEN flag for ath10k devices, as the GLUON_ATH10K_MESH variable is
sufficient to avoid ath10k devices if desired.

Fixes #864

Generate flashable images for the Archer C7 v2 with current stock firmware
again.

To set the region code, the GLUON_REGION variable must be set to "us" or
"eu" in site.mk or as a make argument.

Fixes #860

and add documentation