That should be:
<file first="yes">multiboot.S</file>
or similar.
Modified: branches/xmlbuildsystem/reactos/ntoskrnl/ntoskrnl.xml --- branches/xmlbuildsystem/reactos/ntoskrnl/ntoskrnl.xml 2005-01-19 01:11:43 UTC (rev 13128) +++ branches/xmlbuildsystem/reactos/ntoskrnl/ntoskrnl.xml 2005-01-19 01:37:35 UTC (rev 13129) @@ -26,10 +26,10 @@ </output> </invoke> - <directory name="ke"> <!-- leave this at the top --> + <directory name="ke"> <if property="ARCH" value="i386"> <directory name="i386"> - <file>multiboot.S</file> <!-- leave this at the top --> + <file first>multiboot.S</file> <file>bios.c</file> <file>brkpoint.c</file> <file>bthread.S</file>