luci-base: rpc: add call to enumerate builtin ethernet ports

Add a new luci/getBuiltinEthernetPorts RPC call which returns a consolidated
list of known ethernet ports found in `/etc/board.json`.

Add an x86/64 specific workaround which attempts to enumerate missing
ethernet devices too.

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