net: if: Provide a Kconfig to select default network interface

Default network interface is the first interface if there are multiple
interfaces exist. But this creates an issue when different parts of
the network subsystem wants to choose particular default interface.
At-least with this Kconfig option user can select default network
interface.

Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>