stage-pool/dest-hier-mount.sh: take mount prefix as an argument.
stage-pool/dest-install-fstab.sh: take prefix as an argument.
stage-pool/dest-install-pkg{,-build}.sh: rename pkg -> pkglist.
stage-pool/dest-install-pkg@.sh: add a stage for installing ad-hoc packages and use it in 7-hw-intel-gpu.