bfdd: add IPv4 BFD Echo support that matches RFC

bfdd: add IPv4 BFD Echo support that matches RFC

Modify the existing BFD Echo code to send an Echo message that will
be looped in the peers forwarding plane.   The existing Echo code
only works with other FRR implementations because the Echo packet
must go up to BFD to be turned around and forwarded back to the
local router.    The new BFD Echo code sets the src/dst IP of the
packet to be the local router's IP and sets the dest MAC to be the
peers MAC address.    The peer receives the packet and because it
is not it's IP address it forwards it back to the local router.

Signed-off-by: Lynne Morrison <lynne.morrison@ibm.com>