components/safety_island: Add VLAN support to veth_rpmsg driver

Add VLAN support to the veth_rpmsg driver, handling the device based
on the VLAN tag and exposing the VLAN capability when the Kconfig
CONFIG_NET_VLAN is enabled.

While there, modify the code inside rpmsg_recv_callback(...) to print
any possible error code of net_pkt_write(...) in the error path.

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