Set HOME env var to chroothome

Home directory is configurable but the HOME env was still hardcoded to /builddir
Remove redundant preshell/prechroot hooks from bind_mount and mount

Functions executed by them are already run in preinit which is executed
as part of shell/chroot command
Use mount_root, umount_root and postumount hooks in tmpfs

Those hook which were created for LVM plugin and are also more suitable for
tmpfs. Now it doesn't have problems with unmounting and doesn't rely on
plugin order.