Some binutils versions will drag in an object to satisfy the entry
symbol; some won't.  Try to cope with this exciting variety of
behaviour by ensuring that all entry symbols are unique.

Remove the explicit inclusion of the prefix object on the linker
command line, since the entry symbol now provides all the information
needed to identify the prefix.

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

Flat real mode works perfectly on real hardware, but seems to cause
problems for some hypervisors.  Revert to using 16-bit protected mode
(and returning to real mode with 4GB limits, so as not to break PMM
BIOSes).

Allow the code specific to the .mrom format to continue to assume that
flat real mode works, since this format is specific to real hardware.

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

Add an infrastructure allowing the prefix to provide an open_payload()
method for obtaining out-of-band access to the whole iPXE image.  Add
a mechanism within this infrastructure that allows raw access to the
expansion ROM BAR by temporarily borrowing an address from a suitable
memory BAR on the same PCI card.

For cards that have a memory BAR that is at least as large as their
expansion ROM BAR, this allows large iPXE ROMs to be supported even on
systems where PMM fails, or where option ROM space pressure makes it
impossible to use PMM shrinking.  The BIOS sees only a stub ROM of
approximately 3kB in size; the remainder (which can be well over 64kB)
is loaded only at the time iPXE is invoked.

As a nice side-effect, an iPXE .mrom image will continue to work even
if its PMM-allocated areas are overwritten between initialisation and
invocation.

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