[build] Allow building PCI ROMs with device ID lists

PCI v3.0 supports a "device list" which allows the ROM to claim
support for multiple PCI device IDs (but only a single vendor ID).
Add support for building such ROMs by scanning the build target
element list and incorporating any device IDs into the ROM's device
list header.  For example:

  make bin/8086153a--8086153b.mrom

would build a ROM claiming support for both 8086:153a and 8086:153b.

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

src/arch/i386/prefix/romprefix.S

@@ -91,7 +91,7 @@ pciheader:
 	.ascii	"PCIR"			/* Signature */
 	.word	pci_vendor_id		/* Vendor identification */ 
 	.word	pci_device_id		/* Device identification */
-	.word	0x0000			/* Device list pointer */
+	.word	( pci_devlist - pciheader ) /* Device list pointer */
 	.word	pciheader_len		/* PCI data structure length */
 	.byte	0x03			/* PCI data structure revision */
 	.byte	0x02, 0x00, 0x00	/* Class code */
@@ -107,6 +107,17 @@ pciheader_runtime_length:
 	.equ pciheader_len, . - pciheader
 	.size pciheader, . - pciheader
 
+	/* PCI additional device list (filled in by linker) */
+	.section ".pci_devlist.00000000", "a", @progbits
+pci_devlist:
+	.previous
+	.section ".pci_devlist.ffffffff", "a", @progbits
+pci_devlist_end:
+	.short	0x0000 /* List terminator */
+	.previous
+	/* Ensure that terminator is always present */
+	.reloc pciheader, RELOC_TYPE_NONE, pci_devlist_end
+
 	.section ".zinfo.fixup", "a", @progbits	/* Compressor fixups */
 	.ascii	ZINFO_TYPE_ADxW
 	.long	pciheader_image_length

src/arch/i386/scripts/i386.lds

@@ -41,6 +41,7 @@ SECTIONS {
     .prefix 0x0 : AT ( _prefix_lma ) {
 	_prefix = .;
 	*(.prefix)
+	*(SORT(.pci_devlist.*))
 	*(.prefix.*)
 	_mprefix = .;
     } .bss.prefix (NOLOAD) : AT ( _end_lma ) {