mkarchiso: implement configurable bootstrap tarball compression

mkarchiso: implement configurable bootstrap tarball compression

profiledef.sh gains a new option bootstrap_tarball_compression which is a bash array containing the compression program and its arguments.

Related to #130 (closed).

configs/*/profiledef.sh: set bootstrap_tarball_compression

• baseline will use .tar.zst with zstd -c -T0 --long -19,
• releng will retain .tar.gz with gzip -cn9 for now.

This will later be changed as part of #130 (closed).