luci-mod-status: 29_ports.js: attempt to use getBuiltinEthernetPorts

Try to use the new luci/getBuiltinEthernetPorts RPC call to enumerate known
ports and fall back to manual board.json parsing if the call is unavailable
yet. The fallback code will be dropped in a while when everything settled.

Ref: #6534, #6538
Signed-off-by: Jo-Philipp Wich <jo@mein.io>