Add support for PORTSDIR values other than the default /usr/ports

Currently /usr/ports prefix is hardcoded into pkg. If MAKE_ARGS has
PKG_PORTSDIR defined, it will build pkg with the value of that variable
instead.  e.g. MAKE_ARGS+= PKG_PORTSDIR=${PORTSDIR}

That MAKE_ARGS definition will be added to the ports makefile to
properly support non-standard PORTSDIRs.