HEAD-700: log MASTER_PLATFORM_URL in usb-headnode builds
HEAD-700: log MASTER_PLATFORM_URL in usb-headnode builds

numericillustration/sdc-headnode,numericillustration/sdc-headnode,joyent/sdc-headnode,kaizengarden/sdc-headnode,joyent/sdc-headnode,kaizengarden/sdc-headnode,joyent/sdc-headnode,numericillustration/sdc-headnode,kaizengarden/sdc-headnode,joyent/sdc-headnode,kaizengarden/sdc-headnode