component/firefox: generate a fallback $FONTCONFIG_FILE
Since e5687602 it's instance responsability to generate a fonts.conf if they need fonts, but we realized to late that if there is no /etc/fonts/fonts.conf on the machine firefox does not start.
In recent ERP5 software release, we have a $FONTCONFIG_FILE, but on older versions we don't.
To keep compatibility with old ERP5 software release, we generate a default FONTCONFIG_FILE that will be used as fallback when $FONTCONFIG_FILE environment variable is not set.