fix license notices in source files and license files under GPLv2+

Previously a lot of files were referenced to GNU Public License 2.1,
which never existed.
All copyright attributions are kept the same.

                  <xsd:element name="Generic">
                    <xsd:complexType>
                      %(toolchain_makefile)s
                    </xsd:complexType>
                  </xsd:element>