gateway.d: Add scripts for network configuration

This adds scripts to configure vlan and client network.
This also adds sysctl settings to enable forwarding.

Note:
Devices specific properties are sourced from fff-network package.
This creates a dependency on fff-boardname and fff-network.
These properties should be located elsewhere in the future.

Signed-off-by: Fabian Bläse <fabian@blaese.de>
Reviewed-by: Tim Niemeyer <tim@tn-x.org>
Reviewed-by: Robert Langhammer <rlanghammer@web.de>