Updated use of multiple expansion guard to use a sanitised string for the variable name.
<?xml version="1.0"?>
<!DOCTYPE SystemDefinition SYSTEM "sysdef_1_4_0.dtd" [
<!ENTITY layer_real_source_path "sf/mw/platformtools" >
]>
<SystemDefinition name="platformtools" schema="1.4.0">
<systemModel>
<layer name="mw_layer">
<module name="platformtools">
<unit unitID="tools.platformtools" mrp="" bldFile="&layer_real_source_path;/group" name="platformtools" />
</module>
</layer>
</systemModel>
</SystemDefinition>