[lldp] Add support for the Link Layer Discovery Protocol

[lldp] Add support for the Link Layer Discovery Protocol

Add support for recording LLDP packets and exposing TLV values via the
settings mechanism.  LLDP settings are encoded as

  ${netX.lldp/<prefix>.<type>.<index>.<offset>.<length>}

where

  <type> is the TLV type

  <offset> is the starting offset within the TLV value

  <length> is the length (or zero to read the from <offset> to the end)

  <prefix>, if it has a non-zero value, is the subtype byte string of
  length <offset> to match at the start of the TLV value, up to a
  maximum matched length of 4 bytes

  <index> is the index of the entry matching <type> and <prefix> to be
  accessed, with zero indicating the first matching entry

The <prefix> is designed to accommodate both matching of the OUI
within an organization-specific TLV (e.g. 0x0080c2 for IEEE 802.1
TLVs) and of a subtype byte as found within many TLVs.

This encoding allows most LLDP values to be extracted easily.  For
example

  System name: ${netX.lldp/5.0.0.0:string}

  System description: ${netX.lldp/6.0.0.0:string}

  Port description: ${netX.lldp/4.0.0.0:string}

  Port interface name: ${netX.lldp/5.2.0.1.0:string}

  Chassis MAC address: ${netX.lldp/4.1.0.1.0:hex}

  Management IPv4 address: ${netX.lldp/5.1.8.0.2.4:ipv4}

  Port VLAN ID: ${netX.lldp/0x0080c2.1.127.0.4.2:int16}

  Port VLAN name: ${netX.lldp/0x0080c2.3.127.0.7.0:string}

  Maximum frame size: ${netX.lldp/0x00120f.4.127.0.4.2:uint16}

Originally-implemented-by: Marin Hannache <git@mareo.fr>
Signed-off-by: Michael Brown <mcb30@ipxe.org>