Set the module_platform_id in the yum.conf

conf/yum.conf

@@ -12,6 +12,7 @@ syslog_device=
 install_weak_deps=0
 metadata_expire=3600
 mdpolicy=group:primary
+module_platform_id=$module_platform_id
 
 # repos
 

module_build_service/builder/MockModuleBuilder.py

@@ -339,6 +339,10 @@ class MockModuleBuilder(GenericBuilder):
                 if len(mmds) == 1 and mmds[0].get_name() in conf.base_module_names:
                     for repofile in conf.base_module_repofiles:
                         self._add_repo_from_path(repofile)
+                    # Also set the platform_id.
+                    mmd = mmds[0]
+                    self.yum_conf = self.yum_conf.replace(
+                        "$module_platform_id", "%s:%s" % (mmd.get_name(), mmd.get_stream()))
                 else:
                     # Add repositories defined in repofile to mock config.
                     repofile = source[len("repofile://"):]