system-linux: add option to configure DSA conduit device

system-linux: add option to configure DSA conduit device

Device might have multiple CPU port with DSA based switch and OEM
firmware might set specific port to one CPU port (for example WAN) to
sustain full gigabit traffic with the kernel.

To set them iproute2 tool is currently required.
Add support to set the DSA port conduit directly from network config
using netlink. Example:

config device
            option name 'lan1'
            option conduit 'eth1'

Conduit option refer to the CPU port interface. Invalid option will
simply be ignored and won't be applied similar to what iproute2 does.

Option can also be set in board.json by setting the conduit option.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>