OpenWRT 19.07 enables urngd by default, so haveged is redundant.

Fixes #1775

This new build flag is mandatory for now (it may default to 0 in a future
Gluon version). It may be set to the following values:

* 0 - Do not build any images for deprecated devices.
* upgrade - Only build sysupgrade images for deprecated devices.
* full - Build both sysupgrade and factory images for deprecated devices.

"Other" images are handled like factory images, as they are also used for
the initial installation of Gluon on a device.

Closes: #1436

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.

As PROVIDES can be used to replace real packages now, we don't need the
virtual packages as workaround anymore. This also means that the providing
packages don't need to be added to site.mk explicitly anymore when the
default provider is used.

iptables will be pulled in anyways.

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

 - generalize, to make it more clear that the site config
   needs to be adapted to the local community.
 - remove some host names and public keys used in Lübeck
 - suggest private space mac adresses
 - change fastd method to salsa2012+umac, which is the recommended default
 - add config for a fastd peer reachable by both ipv4 and ipv6
 - order GLUON_SITE_PACKAGES alphabetically