Fix 'install: symlink usr/src/sys -> /sys: File exists' in distributeworld

Follow r334617, specify ${DISTDIR} (by ${INSTALL_DDIR}), '/base' and add
${INSTALLFLAGS} while installing the '/sys' symbolic link.

Reviewed by: bapt (earlier version), markj
Approved by: re (gjb), markj (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D16877