IMGAPI-228: document new CONFIG_AGENT_LOCAL_MANIFEST_DIRS setup.common optional input
IMGAPI-228: document new CONFIG_AGENT_LOCAL_MANIFEST_DIRS setup.common optional input

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