[romprefix] Store boot bus:dev.fn address as autoboot device location

Per the BIOS Boot Specification, the initialization phase of the ROM
is called with the PFA (PCI Function Address) in the %ax register.
The intention is that the ROM code will store that device address
somewhere and use it for booting from that device when the Boot Entry
Vector (BEV) is called.  iPXE does store the PFA, but doesn't use it
to select the boot network device.  This renders BIOS IPL lists fairly
ineffective.

Fix by using the BBS-specified bus:dev.fn address as the autoboot
device location.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/prefix/romprefix.S

 	pushw	$1f
 	lret
 	.section ".text16", "awx", @progbits
-1:	/* Call main() */
+1:
+	/* Retrieve PCI bus:dev.fn */
+	movw	init_pci_busdevfn, %cx
+
+	/* Set up %ds for access to .data16 */
+	movw	%bx, %ds
+
+	/* Store PCI bus:dev.fn */
+	movw	%cx, autoboot_busdevfn
+
+	/* Call main() */
 	pushl	$main
 	pushw	%cs
 	call	prot_call