[romprefix] Report failure cause when unable to open payload

Report the cause of the failure when we are unable to open the .mrom
payload.  There are two possible failure cases:

 - Unable to find a suitable memory BAR to borrow (e.g. if the NIC
   doesn't have a memory BAR that is at least as large as the
   expansion ROM BAR, or if the memory BAR has been assigned a 64-bit
   address which won't fit into the 32-bit expansion ROM BAR).  This
   will be reported as "BABABABA".

 - Unable to find correct ROM image within the BAR.  This will be
   reported as the address (within the borrowed BAR) at which we first
   fail to find a valid 55AA signature.

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

src/arch/i386/prefix/mromprefix.S

@@ -98,6 +98,7 @@ find_mem_bar:
 	cmpw	$PCI_BAR_5, %di
 	jle	1f
 	stc
+	movl	$0xbabababa, %esi	/* Report "No suitable BAR" */
 	jmp	99f
 1:	movw	$4, %bp
 
@@ -157,15 +158,17 @@ find_mem_bar:
 
 	/* Locate our ROM image */
 1:	addr32 es cmpw $0xaa55, (%eax)
-	stc
-	jne	99f
-	addr32 es cmpl $_build_id, build_id(%eax)
 	je	2f
+	stc
+	movl	%eax, %esi		/* Report failure address */
+	jmp	99f
+2:	addr32 es cmpl $_build_id, build_id(%eax)
+	je	3f
 	addr32 es movzbl 2(%eax), %ecx
 	shll	$9, %ecx
 	addl	%ecx, %eax
 	jmp	1b
-2:
+3:
 
 	/* Copy payload to buffer, or set buffer address to BAR address */
 	testl	%esi, %esi