[PATCH| lldpd: use bond slave real mac with locally administered bit set as sour

[PATCH| lldpd: use bond slave real mac with locally administered bit set as sour
ce man in lldp frames sent on bond slaves

By default lldpd sent src mac of zero for all lldp frames sent on bond
slaves. See https://tickets.cumulusnetworks.com/browse/CM-1283

This patch provides a few options to select the source address in lldp
trames sent on bonded slaves.

The problem with using the bond slave real mac in lldp frames
is described in https://github.com/vincentbernat/lldpd/issues/26

This patch provides 3 choices to src mac address selection on bond slaves:

# lldpcli help configure system bond-slave-src-mac-type

-- Set LLDP bond slave src mac type
      real  Real mac
      zero  All zero mac
     local  Real Mac with locally administered bit set

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: Vincent Bernat <bernat@luffy.cx>