RELENG-163: Allow MASTER_PLATFORM_URL to be passed into the build.
RELENG-163: Allow MASTER_PLATFORM_URL to be passed into the build.

Mountain gorilla will use this to specify its release-specific bits dir.

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