If a community uses different vpn providers, they typically
assume the same MTU for the wan device underneath the VPN. As
different VPN providers however have different overhead, the MTU
of the VPN device differs for each provider. Therefore this
commit makes the MTU of the VPN device provider specific.

This has two advantages:
1. The same site.conf can used to bake firmwares for different
   VPN providers (only by selecting a diferent vpn feature in the
   site.mk).
2. We are coming closer to the option of integrating multiple VPN
   providers into one firmware.