drivers: slip: Add LLDP support

Enable LLDP TX support on the slip ethernet driver.
Now when CONFIG_NET_LLDP is enabled, one can easily verify on a sniffer
(i.e. wireshark) that LLDP frames are being sent at the configured
interval with all mandatory TLVs enabled.

Signed-off-by: Jesus Sanchez-Palencia <jesus.sanchez-palencia@intel.com>