The pre-included header file used in builds (.hrh) is normally defined
inside a text file located at $(EPOCROOT)/epoc32/tools/variant/variant.cfg
in each kit.

This is an unnecessary extra file to maintain if you already define all the
other kit properties in an XML file in the $(EPOCROOT)/epoc32/sbs_config
folder.

This change allows kit owners to define the .hrh file name in the XML instead
of using an additional variant.cfg file.

An example of this is shown in $(SBS_HOME)/examples/os_properties.xml
where the VARIANT_HRH parameter is set as follows,

<set name='VARIANT_HRH' value='$(EPOCINCLUDE)/feature_settings.hrh'/>


In the unusual case where you want to build without a .hrh file at all (maybe
to create an epoc32 tree from scratch) there is a variant "nohrh" which
simply sets VARIANT_HRH as above to an empty file. Example usage would be,

sbs -b bootstrap/bld.inf -c tools2.nohrh

Note that, in general, actual code will not build properly without a .hrh
file to configure it.
