Extend python<->PLC global variable description "GlobalDesc" to support Initial value setting.

Rebased from dporopat change "add initial value to runtime.py file", which means :
- GlobalDesc is used by some customized beremiz python runtime
- Those runtimes need to know what is initial value of these special variables

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                  <xsd:element name="Linux">

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                    <xsd:complexType>

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                      %(toolchain_gcc)s

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                    </xsd:complexType>

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                  </xsd:element>