Use generic fields in struct device_description rather than assuming
that the struct device * is contained within a pci_device or
isapnp_device; this assumption is broken when using the undionly
driver.

Add PXENV_UNDI_SET_STATION_ADDRESS.

Implement PXENV_RESTART_TFTP.

Move pxe_boot() code to pxe_image.c

Add IMAGE_PXE to use the new image format framework.  "kernel pxelinux.0"
now works.

PXE code now compiles without errors (though it won't actually work).