base-files: uci-defaults: support requesting untagged switch port configuration

Introduce support for a new [0-9]u@netdev syntax to ucidev_add_switch() to let
board.d network files request default network switch configurations which do
not use tagged CPU ports.

This commit itself has no effect on generated configurations at the moment
since we still emit untagged configurations by default but it allows boards to
opt-out from default tagged configs in case we start emitting tagged settings
by default.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>