treewide: use interface roles as basis for network configuration

With the new role-based interface configuration, it would be better to
rename the wan/wan6 interfaces to uplink/uplink6, but that would cause
unnecessary churn for the firewall configuration, so it is left for a
later update.

As all interfaces with the 'uplink' role are in the br-wan bridge, it is
not possible to assign these to the 'mesh' role independently - instead,
br-wan is added as a mesh interface as soon as a single interface has
both the 'uplink' and 'mesh' roles. The UCI section for this
configuration is now called 'mesh_uplink' instead of 'mesh_wan'.

For all interfaces that have the 'mesh', but not the 'uplink' role a
second configuration 'mesh_other' is created. If there is more than one
such interface, all these interfaces are bridged as well (creating a
bridge 'br-mesh_other'). This replaces the 'mesh_lan' section with its
optional 'br-mesh_lan' bridge, but can also include interfaces that were
not considered "LAN" when interfaces roles are modified (via site.conf
or manually).