[mromprefix] Allow for .mrom images larger than 128kB

The .mrom payload has a code type of 0xff and so the initialisation
length field (single byte at offset 0x02) does not need to be
present.  Use only the PCI header's image length field, which allows
the .mrom payload to be up to 32MB in size.

Inspired-by: Swift Geek <swiftgeek@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/prefix/mromprefix.S

 	movl	%eax, %esi
 	addr32 es movzbl 2(%esi), %ecx
 	shll	$7, %ecx
-	addr32 es movzbl 2(%esi,%ecx,4), %edx
+	addr32 es movzwl mpciheader_image_length(%esi,%ecx,4), %edx
 	shll	$7, %edx
 	addl	%edx, %ecx
 	addr32 es rep movsl
 	.org	0x00
 mromheader:
 	.word	0xaa55			/* BIOS extension signature */
-mromheader_size: .byte 0		/* Size in 512-byte blocks */
 	.org	0x18
 	.word	mpciheader
 	.org	0x1a
 	.word	0
 	.size	mromheader, . - mromheader
 
-	.section ".zinfo.fixup", "a", @progbits	/* Compressor fixups */
-	.ascii	"APPB"
-	.long	mromheader_size
-	.long	512
-	.long	0
-	.previous
-
 mpciheader:
 	.ascii	"PCIR"			/* Signature */
 	.word	pci_vendor_id		/* Vendor identification */