[nbiprefix] Set up real-mode stack before jumping to .text16

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

src/arch/i386/prefix/nbiprefix.S

 	/* Install iPXE */
 	call	install
 
+	/* Set up real-mode stack */
+	movw	%bx, %ss
+	movw	$_estack16, %sp
+
 	/* Jump to .text16 segment */
 	pushw	%ax
 	pushw	$1f