Commit fd0aef9 introduced a typo that caused PMM detection to start at
paragraph 0xe00 rather than 0xe000.  (Detection would still work, since it
would scan until it ran out of base memory, but it would end up scanning
an unnecessarily large portion of base memory.)

Spotted by Sebastian Herbszt <herbszt@gmx.de>.

When the BIOS doesn't support BBS, hooking INT 19 is the only way to add
ourselves as a boot device.  If we have to do this, we should at least
try to chain to the original INT 19 vector if our boot fails.

Idea suggested by Andrew Schran <aschran@google.com>

The ROM prefix now prompts the user to enter the gPXE shell during POST;
this allows for configuring gPXE without needing to attempt to boot from
it.  (It also slows down system boot by three seconds per gPXE ROM, but
hey.)

This is apparently a certain OEM's requirement for option ROMs.

This allows multiple gPXE ROMs in a system to be disambiguated at boot
time; the PCI ID will show up in the boot menu for a BBS-compliant BIOS.

Minor bugfix: 4 x 2 = 8, not 16

When PMM is used, the gPXE image source will no longer be in base memory.
Decompression of .text16 and .data16 can therefore no longer be done in
real mode.

Use BBS installation check to see if we need to hook INT19 even on a PnP
BIOS.

Verify that $PnP signature is paragraph-aligned; bochs/qemu BIOS provides
a dummy $PnP signature with no valid entry point, and deliberately
unaligns the signature to indicate that it is not properly valid.

Print message if INT19 is hooked.

Attempt to use PMM even if BBS check failed.

Move the printing functions from pxeprefix.S into libprefix.S, so they
are available for debug from any prefix.

ROM initialisation vector now attempts to allocate a 2MB block using
PMM.  If successful, it copies the ROM image to this block, then
shrinks the ROM image to allow for more option ROMs.  If unsuccessful,
it leaves the ROM as-is.

ROM BEV now attempts to return to the BIOS, resorting to INT 18 only
if the BIOS stack has been corrupted.

work around limitations in real-mode virtualisation support on Intel
VT-capable chips.

us to round down the size for the relocation copy to the nearest 64kB
(+0x10 bytes); this just happened to work on most machines because the
last 64kB of the image is all-zeroes anyway (it's the .bss).

compressor fixup section.

start of the non-prefix blocks in the loaded image, and rely on the
image ordering.  This should make introducing compression much easier.

"rep movsb".

the Etherboot partition.

handle multiple sectors.

This reverts commit 844d088c51.

(including logical partitions).

This reverts commit 03ca71c38d.

(0x7ef4) tends to really screw up anything that loads at 0x7c00...

ISA 3c509 is currently non-functional, although the EISA (3c509-eisa) and
MCA (3c529) variants should build OK.

None of this code is yet tested.

clean separation between loading and starting, but does mean that more
PXE stacks survive the process.

do it.

allocation.

addresses if the signature checks fail.

we can never return to the PXE stack and must always use INT 18.

Record all information required for populating a struct undi_device.

Make debugging output more human-readable.