We ensure that 'ports.tgz' tarball should have ports/distfiles directory.
At first, ${CHROOTDIR}/usr/ports is empty (rm -rf ports), then extract
ports files from CVS repository. Then if RELEASEDISTFILES is defined,
${CHROOTDIR}/usr/ports/distfiles directory is created by cp(1). However,
if you don't specify RELEASEDISTFILES, there is no chance to create
${CHROOTDIR}/usr/ports/distfiles directory.
Submitted by: {ushi,tora}.jp.FreeBSD.org
MFC after: 5 days