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