gre: Generic Routing Encapsulation package support

gre: Generic Routing Encapsulation package support

The package supports Generic Routing Encapsulation support by registering following protocol kinds:
    -gre
    -gretap
    -grev6
    -grev6tap

Following options are valid for gre and gretap kinds:
    -ipaddr
    -peeraddr
    -df
    -mtu
    -ttl
    -tunlink
    -zone
    -ikey
    -okey
    -icsum
    -ocsum
    -iseqno
    -oseqno

The gretap kind supports additionally the network option

Following options are valid for grev6 and grev6tap kinds:
    -ip6addr
    -peer6addr
    -weakif
    -mtu
    -ttl
    -tunlink
    -zone
    -ikey
    -okey
    -icsum
    -ocsum
    -iseqno
    -oseqno

The grev6tap kind supports additionally the network option

Typical network config for a GREv4 tunnel :

config interface 'gre'
        option peeraddr '172.16.18.240'
        option mtu '1400'
        option proto 'gre'
        option tunlink 'wan'
        option zone 'tunnel'

Typical network config for a GREv4 tap tunnel :

config interface 'gretap'
        option peeraddr '195.207.5.79'
        option mtu '1400'
        option proto 'gretap'
        option zone 'tunnel'
        option tunlink 'wan'
        option network 'wlan_ap'

I added myself as maintainer for the moment; feel free to change.

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@41897 3c298f89-4303-0410-b956-a3cf2f4a3e73