[romprefix] Report a pessimistic runtime size estimate

PCI3.0 allows us to report a "runtime size" which can be smaller than
the actual ROM size.  On systems that support PMM our runtime size
will be small (~2.5kB), which helps to conserve the limited option ROM
space.  However, there is no guarantee that the PMM allocation will
succeed, and so we need to report the worst-case runtime size in the
PCI header.

Move the "shrunk ROM size" field from the PCI header to a new "iPXE
ROM header", allowing it to be accessed by ROM-manipulation utilities
such as disrom.pl.

Reported-by: Anton D. Kachalov <mouse@yandex-team.ru>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/prefix/romprefix.S

@@ -63,6 +63,8 @@ romheader_size:	.byte 0			/* Size in 512-byte blocks */
 	jmp	init			/* Initialisation vector */
 checksum:
 	.byte	0
+	.org	0x10
+	.word	ipxeheader
 	.org	0x16
 	.word	undiheader
 	.org	0x18
@@ -78,9 +80,6 @@ checksum:
 	.long	0
 	.previous
 
-build_id:
-	.long	_build_id		/* Randomly-generated build ID */
-
 pciheader:
 	.ascii	"PCIR"			/* Signature */
 	.word	pci_vendor_id		/* Vendor identification */ 
@@ -106,7 +105,7 @@ pciheader_runtime_length:
 	.long	pciheader_image_length
 	.long	512
 	.long	0
-	.ascii	"ADHW"
+	.ascii	ZINFO_TYPE_ADxW
 	.long	pciheader_runtime_length
 	.long	512
 	.long	0
@@ -175,6 +174,25 @@ undiheader:
 	.equ undiheader_len, . - undiheader
 	.size undiheader, . - undiheader
 
+ipxeheader:
+	.ascii	"iPXE"			/* Signature */
+	.byte	ipxeheader_len		/* Length of structure */
+	.byte	0			/* Checksum */
+shrunk_rom_size:
+	.byte	0			/* Shrunk size (in 512-byte blocks) */
+	.byte	0			/* Reserved */
+build_id:
+	.long	_build_id		/* Randomly-generated build ID */
+	.equ ipxeheader_len, . - ipxeheader
+	.size ipxeheader, . - ipxeheader
+
+	.section ".zinfo.fixup", "a", @progbits	/* Compressor fixups */
+	.ascii	"ADHB"
+	.long	shrunk_rom_size
+	.long	512
+	.long	0
+	.previous
+
 /* Initialisation (called once during POST)
  *
  * Determine whether or not this is a PnP system via a signature
@@ -354,7 +372,7 @@ pmm_scan:
 	addr32 rep movsl	/* PMM presence implies flat real mode */
 	popw	%es
 	/* Shrink ROM */
-	movw	pciheader_runtime_length, %ax
+	movb	shrunk_rom_size, %al
 	movb	%al, romheader_size
 1:	/* Allocate decompression PMM block.  Round up the size to the
 	 * nearest 128kB and use the size within the PMM handle; this