functions/fs: create_fs: use setconf -S to emit `declare -gA` for the MIG_FS_* associative arrays.

Bash apparently requires an explicit declaration in order for the array
to become associative.
stage-pool/dest-install-pkg-build@.sh: rip out 'pkglist' logic remained from dest-install-pkglist-build.sh.
stage-pool/dest-install-pkg{,list}-build: unify builder invocation in functions.sh and make BUILD_MODE adjustable.