Add FILE_LICENCE declarations to almost all files that make up the
various standard builds of gPXE.

NetBSD kernels are multiboot ELF kernels with an entry point
incorrectly specified as a virtual address rather than a physical
address.

Work around this by looking for the segment that could plausibly
contain the entry point address (interpreted as either a physical or
virtual address), and using that to determine the eventual physical
entry point.

In the event of any ambiguity, precedence is given to interpretation
of the entry point as a physical address.

This brings us in to line with Linux definitions, and also simplifies
adding x86_64 support since both platforms have 2-byte shorts, 4-byte
ints and 8-byte long longs.

Delete ELF as a generic image type.  The method for invoking an
ELF-based image (as well as any tables that must be set up to allow it
to boot) will always depend on the specific architecture.  core/elf.c
now only provides the elf_load() function, to avoid duplicating
functionality between ELF-based image types.

Add arch/i386/image/elfboot.c, to handle the generic case of 32-bit
x86 ELF images.  We don't currently set up any multiboot tables, ELF
notes, etc.  This seems to be sufficient for loading kernels generated
using both wraplinux and coreboot's mkelfImage.

Note that while Etherboot 5.4 allowed ELF images to return, we don't.
There is no callback mechanism for the loaded image to shut down gPXE,
which means that we have to shut down before invoking the image.  This
means that we lose device state, protection against being trampled on,
etc.  It is not safe to continue afterwards.

userptr_add().

refer to them by name from the command line, or build them into a
multiboot module list.

Use setting image->type to disambiguate between "not my image" and "bad
image"; this avoids relying on specific values of the error code.

standard and so don't belong in include/elf.h

generic ELF loader, to be used by the multiboot code.