PXE is a catch-all image format with no signature checks.  If an
unsupported image file is loaded, it will be treated as a PXE image.  In
most cases, the image will be too large to be loaded as a PXE image (which
has to fit in base memory), so the error returned to the user will be that
the segment could not fit within the memory region.

Add an explicit check to pxe_image.c to reject images larger than base
memory with ENOEXEC.

Add ENOEXEC to the error string table.

list.

Added FEATURE() macros to most relevant (non-driver) files.

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).