In multidomain setups, VXLAN is enabled by default, but can be disabled in
domain configs using the mesh/vxlan option. In single domain setups, the
mesh/vxlan option is mandatory.
The UCI option for legacy mode is removed.
Fixes#1364
Our VXLAN setup was changed to accept VXLAN packets without checksum almost
2 months ago, so we can disable sending the checksums now as well. Slightly
improves performance.
Also disabling TX checksums and not only allowing incoming packets without
checksum will provide another small speedup. As doing so would break wired
meshing with VXLAN-enabled nodes that require non-zero checksums, we will
wait a few days before this step.
