(Ab)use the "ident" field in transmitted IPv4 packets to convey
metadata about the network device.  In particular:

    bits 0-3 represent the low bits of the "RX" good packet counter
    bits 4-7 represent the low bits of the "RXE" bad packet counter
    bits 8-15 represent the transmitted packet sequence number

This allows some relevant information about the internal state of the
network device to be read out from a packet trace from a non-debug
build of iPXE.  In particular, it allows a packet trace containing
packets transmitted by iPXE to indicate whether or not any packets
have been received by iPXE.

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