[romprefix] Allow .mrom image to be placed anywhere within the BAR

A .mrom image currently assumes that it is the first image within the
expansion ROM BAR, which may not be correct when multiple images are
present.

Fix by scanning through the BAR until we locate an image matching our
build ID.

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

src/arch/i386/prefix/mromprefix.S

 	movw	$PCI_BAR_EXPROM, %di
 	call	pci_write_config_dword
 
+	/* Locate our ROM image */
+1:	addr32 es cmpw $0xaa55, (%eax)
+	stc
+	jne	99f
+	addr32 es cmpl $_build_id, build_id(%eax)
+	je	2f
+	addr32 es movzbl 2(%eax), %ecx
+	shll	$9, %ecx
+	addl	%ecx, %eax
+	jmp	1b
+2:
+
 	/* Copy payload to buffer, or set buffer address to BAR address */
 	testl	%esi, %esi
 	jz	1f