add module for ipv4 uplink via gre tunnel and bgp

as freifunk rheinland offers it to communities
Merge branch 'master' into v4uplink
Merge branch 'master' into v4uplink
add check gateway resources for vpn and uplink modules
uplink: correct includes
uplink: correctly announce hostroute for nat ip
Revert "uplink: correctly announce hostroute for nat ip"

This reverts commit f44017ede030889001a404a41858267f090fc44d.
disable rp_filter

otherwise the ipv4 uplink does not work
Revert "disable rp_filter"

This reverts commit f42f66d28174886af258653669c615d9d12731e5.
disable rp_filter per device
uplink: use new network functions
uplink: ensure clamp-mss rule is first in forward chain
uplink: use new network functions v2

sorry, forgot something the first time
Merge branch 'master' into v4uplink
uplink: use nat parameter directly
uplink: use template instead of inline template
uplink: add to readme
Merge pull request #52 from ohrensessel/v4uplink

add module for ipv4 uplink via gre tunnel and bgp