[RELENG-244] a PKGSRC_DIR (or equiv) to usb-headnode build and used by MG: would speed up MG usb-headnode builds a lot
[RELENG-244] a PKGSRC_DIR (or equiv) to usb-headnode build and used by MG: would speed up MG usb-headnode builds a lot

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