net - Support ping reply when processing net-loop

Add ICMP_ECHO_REQUEST packet support by responding with a ICMP_ECHO_REPLY.

This permits the ping command to test the phy interface when the phy
is put in loopback mode (typically by setting register 0 bit 14).

It also allows the port to respond to an external ping when u-boot is
processing some other net command (such as tftp).  This is useful when
tftp appears to hang.

Signed-off-by: Ed Swarthout <Ed.Swarthout@freescale.com>
Signed-off-by: Ben Warren <bwarren@qstreams.com>