SVGHMI: changes that should fix some corner cases about unlinking and discardable element that still didn't show until now.
- list that would have been declared in some groups could have been discarded.
- elements to be unlinked were comuted based on filtering $hmi_elements, whereas $hmi_widget already filtered list

          <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"/>