Output descriptions to components in generic netlist.

Also hooks up the Python netlist reader to the descriptions so
that they appear correctly in BOMs.  (The BOM generators used
to always get the root component's description rather than the
alias's description.)

Fixes: lp:1774358
* https://bugs.launchpad.net/kicad/+bug/1774358

eeschema/netlist_exporters/netlist_exporter_generic.cpp

@@ -246,6 +246,8 @@ XNODE* NETLIST_EXPORTER_GENERIC::makeComponents()
             // We only want the symbol name, not the full LIB_ID.
             xlibsource->AddAttribute( "part", comp->GetLibId().GetLibItemName() );
 
+            xlibsource->AddAttribute( "description", comp->GetAliasDescription() );
+
             XNODE* xsheetpath;
 
             xcomp->AddChild( xsheetpath = node( "sheetpath" ) );

eeschema/plugins/python_scripts/kicad_netlist_reader.py

@@ -408,10 +408,7 @@ class comp():
         return self.element.get("tstamp")
 
     def getDescription(self):
-        if not self.libpart:
-            return ""
-
-        return self.libpart.getDescription()
+        return self.element.get("libsource", "description")
 
 
 class netlist():