[pxe] Fill in UNDIROMID pointer in !PXE structure

IBM's iSCSI Firmware Initiator checks the UNDIROMID pointer in the
!PXE structure that gets created by the UNDI loader.  We didn't
previously fill this value in.

src/arch/i386/prefix/romprefix.S

@@ -112,7 +112,8 @@ prodstr_separator:
 prodstr_pci_id:
 	.asciz	"xx:xx.x)"		/* Filled in by init code */
 	.size prodstr, . - prodstr
-	
+
+	.globl	undiheader	
 undiheader:
 	.ascii	"UNDI"			/* Signature */
 	.byte	undiheader_len		/* Length of structure */
@@ -524,18 +525,22 @@ undiloader:
 	/* Save registers */
 	pushl	%esi
 	pushl	%edi
+	pushw	%ds
 	pushw	%es
 	pushw	%bx
+	/* ROM segment address to %ds */
+	pushw	%cs
+	popw	%ds
 	/* UNDI loader parameter structure address into %es:%di */
 	movw	%sp, %bx
-	movw	%ss:16(%bx), %di
-	movw	%ss:18(%bx), %es
+	movw	%ss:18(%bx), %di
+	movw	%ss:20(%bx), %es
 	/* Install to specified real-mode addresses */
 	pushw	%di
 	movw	%es:12(%di), %bx
 	movw	%es:14(%di), %ax
-	movl	%cs:image_source, %esi
-	movl	%cs:decompress_to, %edi
+	movl	image_source, %esi
+	movl	decompress_to, %edi
 	call	install_prealloc
 	popw	%di
 	/* Call UNDI loader C code */
@@ -550,6 +555,7 @@ undiloader:
 	/* Restore registers and return */
 	popw	%bx
 	popw	%es
+	popw	%ds
 	popl	%edi
 	popl	%esi
 	lret