rescue: belatedly add zfsbootcfg

nextboot.sh uses zfsbootcfg to enable nextboot functionality for ZFS,
but zfsbootcfg was never added. Add it now since the nextboot binary
that replaced the script also uses it via system.

Sponsored by: Netflix
Reviewed by: kevans, kib, emaste
Differential Revision: https://reviews.freebsd.org/D43844