[ethernet] Add minimal support for receiving LLC frames

[ethernet] Add minimal support for receiving LLC frames

In some Ethernet framing variants the two-byte protocol field is used
as a length, with the Ethernet header being followed by an IEEE 802.2
LLC header.  The first two bytes of the LLC header are the DSAP and
SSAP.

If the received Ethernet packet appears to use this framing, then
interpret the two-byte DSAP and SSAP as being the network-layer
protocol.  This allows support for receiving Spanning Tree Protocol
frames (which use an LLC header with {DSAP,SSAP}=0x4242) to be added
without requiring a full LLC protocol layer.

Signed-off-by: Michael Brown <mcb30@ipxe.org>