br-client: convert option ifname to list ifname

Convert option ifname in br-client to use a list instead. This
simplifies adding and remove interfaces:

uci:add_to_set("network", "client", "ifname", "eth0")
uci:remove_from_set("network", "client", "ifname", "eth0")

An option ifname will be automatically converted to a list when
performing an upgrade.

Packages affected: gluon-mesh-batman-adv-core, gluon-luci-portconfig