Add OPC-UA simple synchronous client extension.

This extension uses python-opcua to growse server nodes, and generates runtime C code that use open62541.
- python-opcua must be available in python path (i.e. "import opcua" must succeed). Can be installed from pypi with "python2 -m pip install opcua --user" .
- at build time, open62541 is expected to be build in "open62541" directory, aside "beremiz" directory.

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                  <xsd:element name="Xenomai">

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                    <xsd:complexType>

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                      %(toolchain_gcc)s

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                      <xsd:attribute name="XenoConfig" type="xsd:string" use="optional" default="xeno-config"/>

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                    </xsd:complexType>

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                  </xsd:element>