This code largely inspired by tap.c.  Allows for testing iPXE on real
NICs from within Linux.  For example:

  make bin-x86_64-linux/af_packet.linux
  valgrind ./bin-x86_64-linux/af_packet.linux --net af_packet,if=eth3

Tested as x86_64 and i386 binary.

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