Remove duplicate files in zipped exports

The zipping function of the exporters would unconditionally add all
files scanned by scan resources to a zip, including all of the files
associated with each feature. This would conflict with
`build_api.scan_resources` adding all of the files to the resources
object that correspond to the enabled features. To resolve this
difference in behavior, I made the zipping function oblivious to
features and had the upper level function, which has access to the
target configuration, do the proper merging.

tools/project_api.py

@@ -104,8 +104,7 @@ def zip_export(file_name, prefix, resources, project_files, inc_repos):
     with zipfile.ZipFile(file_name, "w") as zip_file:
         for prj_file in project_files:
             zip_file.write(prj_file, join(prefix, basename(prj_file)))
-        for loc, resource in resources.iteritems():
-            for res in [resource] + resource.features.values():
+        for loc, res in resources.iteritems():
             to_zip = (
                 res.headers + res.s_sources + res.c_sources +\
                 res.cpp_sources + res.libraries + res.hex_files + \
@@ -223,8 +222,13 @@ def export_project(src_paths, export_path, target, ide, libraries_paths=None,
                                              macros=macros)
     files.append(config_header)
     if zip_proj:
+        for resource in resource_dict.values():
+            for label, res in resource.features.iteritems():
+                if label not in toolchain.target.features:
+                    resource.add(res)
         if isinstance(zip_proj, basestring):
-            zip_export(join(export_path, zip_proj), name, resource_dict, files, inc_repos)
+            zip_export(join(export_path, zip_proj), name, resource_dict, files,
+                       inc_repos)
         else:
             zip_export(zip_proj, name, resource_dict, files, inc_repos)