stage-list-pool/5-bootloader/5-generate: perform grub-install before grub-mkconfig.

Next commit will make grub-mkconfig stage use runtime config data from
grub-install, so sort out dependencies.
functions/fs: register_fs: fix typo-bug in setconf call.
functions/runner: bind-mount config directories instead of copying.

This is to support runtime config alteration from the chroot stages.
stage-pool/dest-install-grub{-efi,@}.sh: add a variant of GRUB2 install procedure for UEFI

Also report config prefix from the old BIOS stage.
stage-pool/dest-generate-grub.sh: honor runtime-set config file prefix.