Fix failing PLC build when building freshly created project that use default libraries blocks. When controller loads libraries according to attibutes in project files (BeremizRoot/Enable_{libname}_Library), use known default if attribute is missing.

          <xsd:attribute name="Command" type="xsd:string" use="optional" default="make -C %(buildpath)s all BEREMIZSRC=%(src)s BEREMIZCFLAGS=%(cflags)s MD5=%(md5)s USE_BEREMIZ=1 FROM_BEREMIZ=1"/>