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.

                  <xsd:element name="Xenomai">
                    <xsd:complexType>
                      %(toolchain_gcc)s
                      <xsd:attribute name="XenoConfig" type="xsd:string" use="optional" default="xeno-config"/>
                    </xsd:complexType>
                  </xsd:element>