The new configuration generates sections iface_single/lan/wan in
/etc/config/gluon. These sections usually refer to a sysconfig-controlled
interface list, but adding custom sections with verbatim interfaces names
is also possible.

Each interface section contains a list of roles. The supported roles are
'client', 'uplink' and 'mesh'. Multiple roles can be configured on the
same interface (for example the old 'mesh_on_wan' setting would become
'uplink'+'mesh').

'client' is subsumed by any other role configured on the same interface
('client'+'mesh' is equivalent to 'mesh'). This property is important, as
it allows the Wired Mesh settings in gluon-web-network to simply add and
remove the mesh role without having to care what other roles are set -
so in the default setup, this would switch between 'client' and
'client'+'mesh' for the LAN interface.

By default, the WAN interface has role 'uplink' and the LAN interface
'client'; if only a single interface exists, the roles from the WAN
interface are used by default. The default for each of the three
interfaces (WAN/LAN/single) can be changed separated in site.conf,
superseding the old mesh_on_wan, mesh_on_lan and single_as_lan settings.