net: config: Add VLAN identifier as network initial configuration

Add a new Kconfig parameter NET_CONFIG_MY_VLAN_ID as initial network
configuration to enable users to set VLAN identifier at startup.

Add a new setup_vlan(...) function to setup the VLAN identifier in
the device, the call have an effect only when NET_CONFIG_MY_VLAN_ID
is above zero.

Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>