add software/erp5/software-erp5-component.cfg.

so that we don't need to differentiate software/erp5/software.cfg in erp5-component branch.

software/erp5/software-erp5-component.cfg

+# This file is for building ERP5 software release using erp5-component
+# branch.
+
+[buildout]
+extends =
+  software.cfg
+
+[slapos.cookbook-repository]
+branch = erp5-component
+
+[erp5]
+branch = erp5-component