[RELENG-239] Make sure that 'coal' is the default for build-image
[RELENG-239] Make sure that 'coal' is the default for build-image

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